36 BIOLOGY OF THE LABORATORY MOUSE 



cells. Perhaps this indicates that the entoderm cells produce it. In any 

 case it soon becomes continuous over the entire inner surface of the trophec- 

 toderm. The fully developed membrane is of uniform thickness and, as can 

 be demonstrated by dissection, surprisingly tough for so delicate a structure. 

 Though non-living, it possesses the surprising property of being able to 

 increase its area to keep pace with the growth of the embryo. Presumably 

 this capacity for growth is dependent on the entoderm cells which are 

 distributed at quite regular intervals over its entire inner surface. 



The amnion. — The early stages of the development of the amnion have 

 been described. Owing to the inversion of the germ layers, the amniotic 

 folds have only a short distance to grow, and amnion formation is conse- 

 quently precocious in the mouse as compared with most other mammals 

 (Figs. qB and E). For the same reason, the area of the amnion at first is 

 small. It expands rapidly, however, to accommodate the growing embryo 

 and by 8 days it forms a large sac over the embryo's entire dorsal surface 

 (Figs. 2 2 and 25C). In the later stages of development the embryo floats 

 free in the amniotic cavity attached only by the umbilical cord. 



The yolk-sac. — The mammalian ovum contains virtually no yolk. The 

 mammals are, however, descended from reptilian ancestors in whose eggs 

 yolk was abundant, and this long period in their evolutionary history has 

 left an indelible, impress on mammalian development. Most striking, per- 

 haps, is the development of a yolk-sac so similar in many details to the 

 reptilian yolk-sac as to be unmistakably homologous. As is often the case 

 with vestigial structures, this has been modified in different ways in the 

 different groups of animals that have inherited it. In the rodents it gives 

 rise to a membrane enveloping the embryo and possessing the dual function 

 of protection and, during the middle stages of development, of absorbing 

 nourishment from the mother. 



The yolk cavity of the mouse may be defined as the cavity derived from 

 the original segmentation cavity or blastocoele and lying between the egg 

 cylinder and Reichert's membrane (Fig. 19A). The yolk-sac is only a part 

 of the boundary of this cavity; namely, that middle portion of the egg cylin- 

 der wall which is composed of mesoderm and entoderm, or in other words, 

 of extra-embryonic splanchnopleure.* 



* It should be noted that in many mammals, e.g., the pig, the allantois as well as 

 the yolk sac are derived from splanchnopleure. This is not the case in the mouse. 

 In this species the yolk-sac, as we are using the term, and the extra-embryonic 

 splanchnopleure are identical. 



