EXISTING ORGANISMS— EMBRYOLOGY 47 



and covered with mesoderm. The intimate association of this 

 sac with the yolk has given rise to the name yolk sac. Last of 

 all an evagination similar to the yolk sac in structure is formed 

 from the hind gut, expands, and in contact with the serosa 

 provides for respiration in the chick. This is the allantois. 



Mammalian Homologies. In the mammalian em])r3'o all of these 

 things are found. The first sul^divisions in man produce a hollow 

 sphere which later corresponds to the outer ectodermal layer of 

 the serosa. A mass of cells buds off inside of this, which in turn 

 produces a second mass ; these are ectoderm and endoderm respec- 

 tively. Mesodermal tissue fills in the spaces between all three 

 (Fig. 26A). Finally the masses of ectoderm, endoderm and meso- 

 derm split, forming cavities which increase in size until the original 

 ecto- and endodermal masses are connected by a slender stalk 



A B C 



Fig. 27. — Outlines of the limb buds of embryos. A, pig; B, rabbit; C, man. 

 Each shows five rounded prominences which are the first evidence of digits. 



with the outer sphere, now obviously similar to the serosa pre- 

 viously described (Fig. 26B, C, D). In the stalk a diverticulum 

 from the hind gut of the embryo runs out toward the serosa, small, 

 it is true, but with exactly the relationships of the allantois, which 

 it is. The embryo develops from the layers of tissue between the 

 cavities of the ectodermal and endodermal masses, and comes to 

 lie in the first of these, which thus corresponds in all particulars 

 with the amnion of the birds and reptiles, and lastly, although the 

 absence of yolk robs it of its original function of absorbing nourish- 

 ment, the endodermal sac has the connections and structure of the 

 yolk sac. In addition to these homologies we find that in the 

 embryo itself, the neurenteric canal is as well developed as in the 

 lowest chordates. 



Organogeny. General Similarity. After these early steps in 

 the development of vertebrate embryos, the definite structures 



