Wiley

The small pyramidal neurons of layers II and III of the rat parietal cortex have been examined with both Golgi staining and electron microscopy. The cell body contains a large nucleus and only a thin rim of cytoplasm in which the Nissl substance is not well developed. A cap of Nissl substance occurs at the base of the apical dendrite, however, and a number of cisternae of the Golgi apparatus occur in this same location. A centriole is often found here too, so that this region of the neuron represents the cell center. Little Nissl substance occurs in the dendrites, which mainly contain well‐ordered arrays of microtubules.

Projecting from the stem of the apical dendrite and from the secondary dendrites are spines. Basically, the spines are of three types. The most common ones have long, thin stalks and small end bulbs. The least common ones are mushroom‐shaped and have thick stalks ending in large bulbs. The other spines are short and stubby, and have no well‐defined stalk. The distribution of these three types of spines has been determined. Although they are rare, spines may also project from the cell body, which has very few synapses on its surface.

The method of embryonic tissue transplantation was used to confirm the dual origin of avian cranial sensory ganglia, to map precise locations of the anlagen of these sensory neurons, and to identify placodal and neural crest‐derived neurons within ganglia. Segments of neural crest or strips of presumptive placodal ectoderm were excised from chick embryos and replaced with homologous tissues from quail embryos, whose cells contain a heterochromatin marker. Placode‐derived neurons associated with cranial nerves V, VII, IX, and X are located distal to crest‐derived neurons. The generally larger, embryonic placodal neurons are found in the distal portions of both lobes of the trigeminal ganglion, and in the geniculate, petrosal and nodose ganglia. Crest‐derived neurons are found in the proximal trigeminal ganglion and in the combined proximal ganglion of cranial nerves IX and X. Neurons in the vestibular and acoustic ganglia of cranial nerve VIII derive from placodal ectoderm with the exception of a few neural crest‐derived neurons localized to regions within the vestibular ganglion. Schwann sheath cells and satellite cells associated with all these ganglia originate from neural crest. The ganglionic anlagen are arranged in cranial to caudal sequence from the level of the mesencephalon through the third somite. Presumptive placodal ectoderm for the VIIIth, the Vth, and the VIIth, IXth, and Xth ganglia are located in a medial to lateral fashion during early stages of development reflecting, respectively, the dorsolateral, intermediate, and epibranchial positions of these neurogenic placodes.

Portions of the labyrinth or villi of placentas from late pregnancy from nine species in four orders of mammals were examined with the electron microscope. Pronounced patterns of layering of the trophoblast were found in these placentas which were all of the hemochorial type. The laboratory rat, laboratory mouse, hamster, and deer mouse have three layers of trophoblast between the maternal blood space and fetal vessels (hemotrichorial), the rabbit two layers (hemodichorial), the guinea pig and chipmunk one layer (labyrinthine hemomonochorial), and the human and armadillo one layer (villous hemomonochorial).

The outer layer of trophoblast of the hemotrichorial placentas (the layer next to the maternal blood) is cellular, but the next two layers are apparently syncytial and are closely apposed to one another. The outer layer of the rabbit placenta is syncytial, while the inner layer contains some pockets of cells. In all of the hemomonochorial placentas examined, the continuous layer of trophoblast was syncytial. It was found that the surface layer of trophoblast of all the placentas studied is rich in granular endoplasmic reticulum, whereas in subsequent layers this element is less abundant. All of the placentas show both surface and basal modifications of trophoblast, but caveolae (pinocytotic vesicles) were found to be most commonly located in crypts, pockets, or other regions somewhat removed from the surface. It is suggested that an area of relative stasis of maternal plasma may be important for absorption of some substances by the placenta.