THE EARLY EMBRYOLOGY OF THE MOUSE 2>2> 



sheets of mesoderm. It thus forms the base of a U of which the lateral 

 mesoderm forms the sides. By about the four somite stage or slightly later, 

 the coelom extends not only throughout the lateral mesoderm but also as a 

 passage through this anterior mesoderm (Figs. i6 and 29). The coelom, 

 also, is thus U-shaped. The whole posterior portion of the coelom opens 

 laterally into the extra-embryonic coelom (Figs. 17 and 28), The anterior 

 part of the coelom, on the other hand, forming the base of the U and extend- 

 ing as far posteriorly as the second somite, is separated from the extra- 

 embryonic coelom by a partition of mesoderm. Much of this anterior 

 portion of the coelom becomes the pericardial coelom, enclosing the heart. 

 The connection between the anterior and the lateral parts of the coelom is 

 called the pericardial-peritoneal canal (Fig. 28). 



The relations of coelom and extra-embryonic coelom can be studied from 

 the series of sections of a seven somite embryo shown in Figs. 23, A to G. 

 The reader should note, however, that owing to the rapid development of the 

 heart between the four and the seven somite stage, the pericardial coelom 

 is already at this latter stage a more complicated cavity than when it first 

 appeared. 



Reichert's membrane. — The mouse embryo is protected during its 

 development by three extra-embryonic membranes; namely, Reichert's 

 membrane, the amnion and the yolk-sac. There is no essential difference 

 between the amnion of rodents and that of other mammals. Reichert's 

 membrane, on the other hand, is found only in the Rodentia, while the yolk- 

 sac in this order has come to have rather unusual relations to other struc- 

 tures. The chorion, an important fetal membrane in most mammals is 

 present in the mouse but remains small and, as a protective structure, 

 unimportant. 



To follow the development of Reichert's membrane we must go back to 

 the 5!^ day stage (Fig. 8). Except in the region of the ectoplacental cone, 

 the embryo is bounded by the trophcctoderm. This is continuous with the 

 margins of the cone and is separated from the egg cylinder by the yolk 

 cavity. Laterally its cells are in close contact with the maternal decidua, a 

 contact so intimate in fact that in some cases it is impossible to tell whether a 

 given cell is of embr>'onic or maternal origin. Ventrally it stretches across 

 the remains of the uterine lumen, now filled with a degenerating mass of 

 uterine epithelium. On the inner surface of the trophcctoderm are a few 

 widely separated entoderm cells. 



A day later (Fig. 10) these entoderm cells have increased in number and 

 form a uniform though not quite continuous layer over the inner surface of 



