308 PROFESSOR J. COSSAR EWART. 



limb rudiments ; (7) the presence of a broad fin-like expansion at the caudal end of 

 the embryo (fig. 1 1 ) ; (8) the external opening from the posterior end of the spinal 

 cord (fig. 10) ; (9) the broad yolk-stalk connecting the embryo with the yolk-sac 

 (fig. 33) ; and (10) the mouth (fig. 33) between the fronto-nasal process and the first 

 pair of branchial arches (fig. 9). 



2. The Nervous System and Sense Organs. 



At the middle of the third week the nervous system is represented by a medullary 

 groove, shallow at its origin immediately in front of the primitive groove, but deep 

 in the region of the mesodermic somites (fig. 7). This groove communicates by 

 means of a neurenteric canal with the rudiment of the hind-gut. Before the end of 

 the third week the medullary groove is converted into the medullary or neural canal. 

 Though at the end of the third week the neural canal still opens to the exterior at 

 the caudal end (fig. 33), it is closed in front and the cerebral vesicles are already 

 modified to form rudiments of the fore-, mid-, and hind-brain. The fore-brain, 

 separated by a shallow groove externally and a faint ridge internally from the 

 mid-brain, occupies the greater part of the obliquely directed fronto-nasal process 

 (fig. 33). The transverse sections through this process show amongst other things 

 the original cavity of the anterior cerebral vesicle and the primary optic vesicles 

 (fig. 36). The mid-brain, small and indistinctly separated from the hind-brain, has 

 a relatively large oval cavity. In fig. 37 the large cavity of the mid-brain is seen 

 also, the notochord lying between the mid-brain and the diverticulum (Seessel's pouch), 

 which for a time projects from the roof of the fore-gut immediately within the bucco- 

 pharyngeal membrane. 



The hind-brain (figs. 38 to 40), long and tubular, is neither distinctly separated 

 from the mid-brain nor the spinal cord. After curving backwards and downwards 

 the hind-brain forms a dorsal curve which probably marks the beginning of the spinal 

 cord. The chief interest of the hind-brain at the end of the third week is its relation 

 to the otic vesicles. These vesicles (fig. 38) lie close to the hind-brain, nearly on a 

 level with the second branchial pouch.* 



The spinal cord, in the form of an ectodermic tube with a distinct lumen, extends 

 from the hind-brain to the caudal end of the embryo to end in a mass of cells with 

 which the notochord and the cloacal diverticulum are intimately related. The rela- 

 tion of the spinal cord to the notochord and hind-gut, and the opening of its canal 

 to the exterior, are indicated in fig. 33, while in fig. 52 a section of the cord immedi- 

 ately in front of the external opening is represented. From the figures of transverse 

 sections of the embryo it will be observed that the spinal cord assumes a triangular 

 form in the caudal region (figs. 50 to 52), but a more or less oval form throughout 

 the greater part of the trunk (figs. 41 to 49). 



* Owing to the slight obliquity of the section, only the external opening of one of the vesicles is seen in 

 the figure. 



