DEVELOPMENT OF MESODERM. . 295 



stratum of the germ-vesicle is already beginning to extend. (Fig. 77-81, 

 after Bisehoff.) 



Fig. 78. — The same, seen from one side. The letters as in Fig. 77. 



Fig. 79.' — Egg from the uterus of a Eabbit (6 mm. in diameter). The 

 germ -membrane is already to a great extent double-layered (b). The outer 

 egg-membrane (procJiorion) becomes knotty, or wai-ty (a). 



Fig. 80. — The same, seen from one side. The letters as in Fig. 79. 



Fig. 81. — Egg from the uterus of a Rabbit (8 mm. in diameter). Nearly 

 the whole of the germ-membrane vesicle is already double-layered (&) ; only 

 below (at d) there is still only one layer. 



* 



has raised itself from the intestinal germ-vesicle (Fig. 

 79-81 a). 



We need not at present pay any attention either to this 

 outer egg-membrane (prochorion) or to the larger portion of 

 the germ- vesicle, and may turn our full attention to the 

 germ-area (or germ-disc). For it is in this part alone that 

 the important modifications which result in the specializa- 

 tion of the earliest organs first appear. In this respect it is 

 quite immaterial whether we examine the germ-area of a 

 Mammal (e.g. a Rabbit), the germ-disc of a Bird or a Reptile 

 (e.g. a Lizard or a Tortoise). For in all members of the three 

 higher vertebrate classes, all called Amnion-animals, the 

 germinal processes which immediately follow are essentially 

 alike. In this respect Man is like the Rabbit, the Dog, the 

 Ox, etc. ; and in all these Mammals the germ-area undergoes 

 essentially the same modifications as in Birds and Reptiles. 

 It is in the Chick that these have been chiefly and most 

 accurately traced, for any requisite number of incubated 

 hen's eggs, in all stages, can always be obtained. Within 

 a few hours from the beginning of incubation the cir- 

 cular germ-disc of the Chick also passes from a two- 

 layered to a three-layered stage, in consequence of the 

 development of the mesoderm between the exoderm and 

 the entoderm. 



