DEVELOPMENT OF LEPUS 



229 



blastocyst becomes attached by means of its trophoblast. 

 This process is called implantation. 



Formation of the Embryo. — The blastocyst enlarges and 

 expands in the cavity of the uterus. The cells of the inner 

 mass become arranged in the form of a flattened disc, imme- 

 diately beneath the trophoblast. This disc is known as the 

 embryonic plate. At the same time, the inner mass gives 

 rise to a layer of cells which grow as an epithelium lining the 



lam- 



Fig. 109. — Diagrams showing the formation of the amnion in the rabbit. 

 (From Jenkinson, after van Beneden.) 



The earlier stage is on the left ; the later stage on the right. Since the 

 cells of Rauber have disappeared, the embryo is at the surface of the blasto- 

 cyst until the amnion has formed, all, allantois ; atr, region of the tropho- 

 blast where the allantoic placenta will be formed ; c, extra-embryonic 

 ccelom ; e, embryo ; ham, head amniotic fold ; otr, region of the trophoblast 

 where the omphaloidean placenta is formed ; st, sinus terminalis (blood- 

 vessel) of the area vasculosa ; tarn, tail amniotic fold, ys, yolk-sac. 



inner surface of the trophoblast. This layer is endoderm 

 (also called the " lower layer "), and the cavity which it encloses 

 represents the yolk-sac of the chick. Here, however, there is 

 no yolk, and the yolk-sac is consequently empty. 



The cells of the trophoblast immediately overlying the 

 embryonic plate (the cells of Rauber) disappear, and the 

 embryonic plate thus comes to the surface of the trophoblast. 

 A primitive streak is formed in the centre of the embryonic 

 plate, and, as in the chick, it proliferates mesodermal cells 



