HUBRECHT. PC. CE. 
al en ec uv b 
Fig. 55. The same of the opossum (after Selenka); 4 blastopore, av 
umbilical vesicle, ec ectoderm, ez entoderm, @Z albuminiferous layer, ¢7 tropho- 
blast. — Fig. 56, 57 and 58. Three longitudinal sections through on early blasto- 
cyst of the shrew (Sorex) of wich the embryonic shield is traced in the dia- 
gram of fig. 59. Fig. 56 and 57 are two succeeding sections through the posterior 
region where a rudimentary blastopore /2) pierces the embryonic shield, sepa- 
rating the proliferating ectodermal region pw (protochordal wedge) from the yet 
further posterior ectodermal region which will give rise to the ventral mesoblast 
(vm), in which the posterior coelom will take its origin in crescent-shape as in- 
dicated in diagram 61 and 100. Fig. 58 is the longitudinal section through the 
entodermal proliferation pp of Fig. 59. Fig. 100 gives a longitudinal section 
through the posterior crescentic coelom of fig. 61. — Fig. 59. Superficial aspect 
of the early ectodermal shield corresponding to the three preceding figures. 
The region of the protochordal wedge is indicated by the posterior white, that 
of the protochordal plate by the anterior shaded space. 
