236 THE CONTINUITY OF THE RACE 



by the development of a third fundamental cell layer, the mesoderm. At 

 this point the similarity of development that is common to the embryos 

 of all metazoans (except sponges) becomes less evident, and the embryos 

 of various phyla begin to develop the diverse patterns of symmetry and 

 organization that distinguish their respective groups. Here we can con- 

 sider only very briefly the mesoderm formation in Amphioxus and the 

 frog. 



In Amphioxus, gastrulation is accompanied by an elongation of the 

 embryo, which is beginning to show bilateral symmetry. On either side 

 of the middorsal line of the elongating (head to tail) axis of the body 

 the entoderm begins to evaginate (fold out) a linear series of connected 



Fig. 15.15. The early development of the frog, showing the effects of a moderate concentra- 

 tion of yolk at the vegetative pole. A, beginning of second cleavage division. B, third cleav- 

 age division completed. C, section of blastula. D, section of gastrula. The projecting mass 

 of cells is the yolk plug. (Modified from Turtoz chart, courtesy General Biological Supply 

 House, Inc.) 



pouches. These outgrowths become the mesoderm; they rapidly enlarge 

 and are soon cut off from the entoderm tube, which "heals" together 

 to show no traces of its former connection with the mesodermal pouches. 

 The right and left mesodermal tubes grow rapidly and soon meet and 

 fuse along the mid-ventral line of the embryo, thus completely separating 

 the ectoderm of the outer body wall from the entoderm of the gut. The 

 cavity of the mesodermal tubes forms the coelom, or true body cavity, 

 which is thus lined with mesoderm and forms the outer space of the 

 " tube- within-a- tube" body structure. 



Mesoderm formation in the vertebrates is clearly comparable to that 

 in Amphioxus, but it is usually modified or distorted, either by the yolk- 

 laden condition of the entodermal cells or, in the mammals, as a result of 

 descent from ancestors that had such yolk-laden eggs. 



The Primary "Germ Layers" 



The ectoderm, entoderm and mesoderm constitute the so-called "pri- 

 mary" or "germ" layers, from which all the structures of triploblastic 

 animals are developed. In all the higher metazoans the differentiation 

 of these layers constitutes but a very small fraction of embryonic develop- 



