EARLY MAMMALIAN DEVELOPMENT. 91 



condition is characteristic of a number of rodents, including the rat, guinea- 

 pig and mouse. The cluster of cells on one side of the blastocyst increases 

 in size by proliferation of the cells and enlargement of individual members of 

 the group so that it projects outward somewhat and inward into the cavity of 

 the vesicle (Fig. 61, A). Within the cluster a small group of cells becomes 

 differentiated (staining more deeply) which represents the rudiment of the 

 ectodermal node. The layer of cells covering this node on the side toward 

 the cavity is the entoderm. The group of cells projecting outward is known 

 as the ectoplacental cone. Following this, the vesicle elongates and the 

 ectodermal node appears to be forced farther into the blastocyst cavity by a 

 group of cells which develops between the node and the ectoplacental cone 

 and which is extraembryonic ectoderm (Fig. 61, B). The entoderm is more 

 extensive, covering the ectodermal node and much of the extraembryonic 

 ectoderm. At this stage it is evident that entoderm is outside of ectoderm, 

 the condition which has given rise to the term 'inversion of the germ layers.' 

 The two layers together constitute the egg-cylinder (of Sobotta). 



As development proceeds there is still further elongation of the vesicle, 

 with concomitant lengthening of the egg-cylinder. Within the ectodermal 

 node a cavity appears, and around this the cells arrange themselves in the 

 form of a simple columnar epithelium. The space is known as the proamni- 

 tic cavity (Fig. 61, C). The original wall of the blastocyst, except at the 

 ectoplacental cone, is drawn out into a thin membrane-like layer. The 

 cone itself is longer. Following this stage a number of discrete spaces appear" 

 among the extraembryonic ectodermal cells and then coalesce to form one 

 continuous cavity which breaks through into and becomes continuous with 

 the proamniotic cavity of the original node (Fig. 61, D). The cells around 

 this new cavity are also arranged as a simple columnar epithelium; but the 

 boundary line between embryonic and extraembryonic ectoderm is still 

 evident. At this time, when the ova are eight days old, there is not yet any 

 indication of bilaterality in the egg-cylinder. 



In slightly later stages, shown in cross section in Fig. 62, A and B the 

 columnar entoderm surrounding the extraembryonic ectoderm exhibits an 

 outer vacuolated zone and an inner granular zone, while the entoderm of the 

 embryonic region becomes flat. A little further along in development a new 

 group of cells appears between embryonic ectoderm and entoderm on one 

 side of the egg-cylinder near the junction between embryonic and extra- 

 embryonic regions (Fig. 63, A). This group of cells spreading out between 

 the other two layers is the early mesoderm, which here is evidently a deriva- 

 tive of embryonic ectoderm since it is not even attached to the entoderm. 

 The area where thg mesoderm appears marks the site of the primitive streak 

 and therefore the caudal end of the future embryo. The proamniotic 



