The Development of the AlHgator 269 



medullary canal is closed both above {mc) and 

 below {mc'). The amnion {a) has about the same 

 appearance as in the more anterior section, but 

 there is here a considerable space, filled with meso- 

 blast imes), between the nervous (;?/) and epidermal 

 {ep) layers of ectoderm. 



Figure iic is twenty sections, about one tenth 

 the length of the embryo, posterior to the one 

 last described. The large mass of overhanging 

 yolk (3^) is still present, as is also the amnion (a), 

 though the latter no longer passes entirely around 

 the embryo ; only the true amnion could be made 

 out. The thickened walls of the medullary canal 

 have reduced that cavity to a narrow, Y-shaped 

 slit (nic). The notochord [nt) is very slender 

 in this region, compared to its diameter farther 

 toward the posterior end. The enteron {ent) is a 

 large cavity, whose wall is made up of loosely 

 arranged cells except around a median, ventral 

 depression where the cells are more compact. 

 This depression may be traced through ten or 

 fifteen sections and may represent the beginning of 

 the thyroid gland, though this point was not 

 worked out with certainty. Surrounding the 

 notochord and enteron is a loose mass of typical, 

 stellate mesoblast cells (ines), which are cleft on 

 either side to form the anterior limit of the body 

 cavity {be). Between the body cavity below 

 and the enteron above, on each side, is a small 

 blood-vessel {bv) which when followed caudad 



