carpenter: development of the oculomotor nerve. 177 



2. Development of the Oculomotor Nerve, the Ciliary Ganglion 

 AND the Abducent Nerve j described by Stages. 



Over fifty series of sections were made from chick embryos of various 

 ages between the sixtieth hour and the seventh day of incubation, the 

 period during which the third and sixth nerves and the ciliary ganglion 

 assume both their distinctive histological characters and the most of 

 their adult anatomical relations. From these series have been selected, 

 for the purposes of the descriptions which follow, those best illustrating 

 the successive steps in the development of the nervous structures with 

 which we are dealing. For convenience in description I have divided 

 the first five days of this period into five stages, which will be described 

 in chronological order. 



Stage I. 



1. Oculomotor Nerve. The earliest indication of the origin of the 

 oculomotor nerve was observed in an embryo of seventy-two and one-half 

 hours' incubation. Tliat the development of this embryo had been ab- 

 normally retarded cannot be doubted, since the third nerve in a more ad- 

 vanced stage was freq\iently met with in embryos of seventy hours and 

 in one case in an embryo of sixty hours. 



Parasagittal sections of the mid-brain at this stage show the thickness 

 of its ventral wall to be about one-eighth the height of the neural canal. 

 Entering into the composition of the medullary wall may be observed 

 the elements first described by His and later studied in such detail by 

 Schaper ('97). Near the internal limiting membrane lie numerous 

 germinative cells in process of division, while the mantle layer, making 

 up the greater part of the wall, is composed of ependymal cells which 

 have assumed a supporting function by developing into a medullary 

 framework, and of the products of the proliferating activity of the germ- 

 inative cells, namely, indifferent cells, which later differentiate into 

 either nervous or supporting elements. The structure of the medullary 

 framework is not well brought out by the vom Eath method. Near 

 the external limiting membrane there is a narrow zone free from nuclei, 

 the marginal veil (" Randschleier "). In addition, at a distance of 

 about 100 micra from the median plane on either side, is to be seen a 

 small group of cells which tlie vom Eath stain renders distinguishable 

 from the other elements of the medullary wall. Each cell shows, at one 

 side of its nucleus, a variable amount of darkly colored cytoplasm, the 

 characteristic feature of a neuroblast. Inasmuch as these neuroblasts 

 VOL. XLViii. — No. 2 12 



