THE EARI.Y EMBRYOLOGY OF THE MOUSE 25 



entodermal proliferation must be recognized. As to the place of origin, 

 there is a certain similarity between the two tissues, one forming at the 

 ventral margin of the inner cell mass, the other near the ventral tip of the 

 egg cylinder, which is, so to speak, simply the inner cell mass grown up. 

 Cell lineage studies might reveal a closer similarity in origin than is super- 

 ficially apparent. 



The archenteron. — At 7^^ days there is a broad depression in the rather 

 thick base of the notochord adjacent to its junction with the primitive 

 streak (Figs. 14D and 15). The depression is a conspicuous landmark at 

 this stage, but it is a transitory structure, the first signs of it appearing at 

 7I4 days and disappearance being complete about twelve hours later. It 

 plays no part in later development and probably is best interpreted as a 

 vestigial structure corresponding to a similar structure that occurs in more 

 marked form in the development of reptiles,* and which in turn can probably 

 be traced back to the archenteron of the lower chordates. On the basis of 

 this probable homology it may be called the archenteron. 



The allantois. — Soon after the exocoelomic cavity becomes well estab- 

 lished, a process begins to grow out into this cavity from the mesoderm at 

 the caudal end of the primitive streak. This is the allantois (Fig. 13), an 

 extra-embryonic, mesodermal structure whose function is to convey blood 

 vessels from the embryo to the placenta where they establish contact with 

 the maternal circulation. In many vertebrates the allantois contains a 

 cavity lined with entoderm and connected with the gut. There is no ento- 

 derm-lined cavity in mice; on the other hand there are numerous small 

 cavities in the mesoderm giving the organ a porous structure. 



After its first appearance at 7I4 days the allantois grows rapidly across 

 the exocoelom in the direction of the ectoplacental cone (Figs. 15 and 16). 

 Meantime the chorion becomes flattened against the base of the cone, 

 constricting the ectoplacental cavity and finally eliminating it altogether. 

 When the allantois makes contact with the chorion at about 8 days, a con- 



* See for example Figs. 21 and 22 of Prentiss and Arey (51). We have found no 

 trace of a neurenteric canal in the mouse, in the sense of a canal passing through the 

 ectoderm and the base of the notochord and connecting amniotic cavity and yolk 

 cavity. However, we have seen in a 7I4 day embryo a very short canal confined to 

 the base of the notochord. The ventral wall was thin, and it may be presumed that 

 it would shortly disappear, giving rise to the depression described above. Sobotta's 

 Fig. 14 (62) shows a canal somewhat similar to the one we have noted, except that our 

 material does not suggest, as his drawing does, that the canal is formed by invagination 

 of the entoderm. Jolly and Ferester-Tadie (26) have figured a section almost identical 

 with ours. 



