THE SPINAL CORD. 



251 



Three of these depressions have been observed in chick em- 

 bryos. They appear in succession ; the first one shortly before 



BF 



SP 



MS 



SP 



FIG. 111. 



FIG. 



FIG. 111. A Chick Embryo at the thirty-sixth hour of incubation; seen from 



the dorsal surface, x 20. 



FIG. 112. A median longitudinal, or sagittal, section of a Chick Embryo at 

 the thirty-sixth hour of incubation, x 20. 



AM", head fold of the amiiion. BF, fore-brain. BH, hind-brain. BM, mid-brain. 

 BO, optic vesicle. CH, uotochord. CP, pericardial cavity. El. auditory pit. GrF. 

 fore-gut, or anterior jiortion of the mesenteron. TT, hypoblast. MS, mesoblastie somite 

 or proto vertebra. NS, spinal cord. NT, neurenteric canal. PS, primitive streak. 

 B.V, ventricular portion of the heart. SO, soiuatopleurc. SP, splaucliuopleure. TA. 

 allaiitois. W, vitelline veins. 



the end of the first day; the second one (Fig. 112, >'T) about 

 the middle of the second day ; and the third one in the course 

 of the third day. 



