THE EARLY EMBRYOLOGY OF THE MOUSE 7 



blastula quickly sets up changes at the implantation site. Within a few 

 hours the epithelium begins to loosen, and its nuclei to show degenerative 

 changes (Fig. 5). Within 15 hours it is sloughed off entirely (Fig. 6). At 

 the same time active growth commences in the mucosa, so that by i day after 

 implantation (5 days after mating) there is an appreciable swelling in the 

 uterus at the implantation site. The swollen mucosa at the implantation 

 site is known as decidua. 



Meanwhile the zona pellucida has been lost from around the egg, perhaps 

 through a process of digestion by means of an enzyme secreted by the uterine 

 mucosa (11), though neither the exact time nor mechanism is thoroughly 

 known. 



Up to the time of implantation there has been no growth in size in the 

 egg. Cleavage has resulted in a division of the egg, originally one large cell, 

 into numerous smaller cells, but little if any new protoplasm has been formed 

 in the process. Beginning with implantation, however, rapid growth com- 

 mences. At first the blastocoele enlarges, while the inner cell mass assumes 

 a flattened cup-shape with the concave face towards the cavity (Fig. 5). 

 In the living condition the blastocoele is probably distended with fluid, and 

 its walls tightly pressed against the uterine epithelium, but in fixed material 

 at this stage there is always some collapse. This initial expansion of the 

 blastocoele requires only a few hours and is quickly followed by a growth of 

 the inner cell mass down into the enlarged cavity (Fig. 6). Blastocoele and 

 inner cell mass both are known thereafter by new names; namely, yolk 

 cavity for the former and egg cylinder for the latter. A comparison of 

 Figs. 7, 8, 10 and 12 will show the rapid growth of the egg cylinder that 

 occurs during the next two' and one-half or three days. 



The formation of the entoderm. — At the same time that the blastocoele 

 begins to enlarge, the inner cell mass can be seen to be composed of two types 

 of cells (Fig. 5). Adjacent to the blastocoele is a single layer of darkly 

 staining cells. This is the entoderm, one of the three primary germ layers. 

 The rest of the blastocyst is composed of ectoderm, divided into the ecto- 

 derm of the inner cell mass, and the trophectoderm, a single celled layer 

 bounding the blastocoele ventrally and laterally. The trophectoderm 

 (troph from the Greek word for nourishment) derives its name from the fact 

 that it probably plays a role in the nourishment of the young embryo. The 

 mesoderm has not yet appeared. 



Very shortly after the first appearance of the entoderm, single cells or 

 strands of cells grow out from its margin down along the inner surface of the 

 trophectoderm. At first these cells are few and widely separated (Figs. 7 



