carpen'ter: development of the oculomotor nerve. 193 



nerve. This accumulation I believe to be composed of young ganglion 

 cells, developing from indifferent cells stranded, as it were, on their way 

 from the neural tube to the ciliary ganglion. Around each nucleus the 

 characteristic deeply staining cytoplasm is in process of development. 

 Whether these cells later disintegrate, or produce secondary ganglia, 

 such as are sometimes found in fishes, amphibians, birds and mammals 

 (Schwalbe, '79 ; Jegorow, '86-87), I do not know. Such an accumula- 

 tion does not occur along the nerve of the opposite side. 



4. Abducent Nerve. The sixth nerve emerges in this stage, as in the 

 preceding, by several roots, crowded with "accompanying" cells; Its 

 nidnlus is not well defined. The roots of the nerve lie about 190 micra 

 from the median plane, and have much the appearance seen in the fol- 

 lowing stage, which is shown in Figure 18 (Plate 6). They unite ventral 

 to the hind-brain to form a straight, unbrauched nerve, which runs hori- 

 zontally cephalad, becoming more attenuated toward its distal end, and 

 terminating in the postero-dorsal extremity of the posterior rectus muscle. 



In the roots of the nerve can be seen evidence of cell migration from 

 the neural tube. The more or less rounded cells found at the proximal 

 end of the nerve pass rapidly into the elongated " accompanying " cells, 

 which lie distally along its fibrils. Many of the " accompanying " cells 

 show mitotic figures. At no place on the abducens is there an accumu- 

 lation of rounded cells like that which in Stage III forms on the oculo- 

 motor the fundament of the ciliary ganglion. In the case of the sixth 

 nerve the indifferent cells which migrate out from the hind-brain appear 

 to develop exclusively into structures with the supporting function, the 

 sheaths of Schwann. 



5. Eye Muscles. In this stage first appears the fundament of the 

 ventral oblique muscle, the most anterior of the four eye muscles which 

 are innervated by the oculomotor nerve. At the same time we find that 

 the free end of the nerve extends beyond the ciliary ganglion as far as 

 this muscle mass. 



The axis of the posterior rectus muscle now points in a postero-dorsal 

 and antero-ventral direction, instead of nearly longitudinally, as in 

 Stage III. The dorsal, anterior and ventral rectus muscles retain their 

 primitive relations in both this stage and the following one ; in the 

 account of the latter they are described and figured, together with the 

 ventral oblique muscle. 



Stage V. 



The conditions characteristic of the fifth stage of development are 

 found in embryos of one hundred and eighteen to one hundred and 

 VOL. XLviii. — No. 2 13 



