REPRODUCTION IO9 



from the body wall so that it is derived actually from epimere mesoderm. 

 The somatic folds which are involved in forming the diaphragm arise far 

 forward and shift back to the definitive position of the organ. This fact 

 accounts for the innervation of the diaphragm by cervical spinal nerves. 



In vertebrates other than mammals there are two divisions of the 

 coelom, the pericardial and the peritoneal (abdominal or pleuroperitoneal) , 

 and the lungs he in the posterior division (Fig. 80). In mammals that 

 part of the coelomic space lying on the cephaHc side of the diaphragm is 

 subdivided into three cavities, the pericardial and the right and left 

 pleural cavity containing the corresponding lobes of the lungs. 



The tail is produced by growth of ectodermal and mesodermal parts 

 backward from the region of the blastopore or neurenteric canal. Growth 

 of the mesoderm keeps pace with that of the neural tube and notochord. 

 The mesoderm forms somites, more or less numerous depending on the 

 length of the tail, but these produce only the segmental striated caudal 

 muscle and the mesenchyme which goes into the formation of skeletal, 

 vascular and connective tissue structures of the tail. 



Effect of Massive Yolk on Organogenesis 



In the description of the earher embryonic stages of reptiles and birds 

 attention has been given to the peculiarities which are imposed upon the 

 earher developmental processes by the massive yolk of the egg. Elasmd- 

 branchs, with their large yolk-laden eggs, exhibit similar peculiarities. 

 Cleavage, gastrulation and the mode of origin of the mesoderm and the 

 notochord are necessarily much affected by the presence of the bulky 

 and inert yolk (Fig. 81). Once the germ layers have been established, 

 however, the development of organs proceeds in vertebrates of all classes 

 with only minor differences in details of the processes. Apparently each 

 germ layer is capable of producing certain structures and no others, and 

 those particular structures arise from that layer in all vertebrates, whether 

 fish or man. Indeed, there is so much evidence of rigid necessity in the 

 germ-layer origin of organs that, in cases where homology of adult organs 

 is in question, the germ-layer relations of those organs are generally 

 regarded as the safest criterion for homology. Yet at early stages of 

 development the embryonic material may not be so rigidly determined. 

 By appropriate operations at sufficiently early stages of embryos, both 

 vertebrate and invertebrate, it has been proved that a certain region of 

 germ material may be caused to produce structures other than those 

 which it would have produced normally. 



While a particular organ develops in essentially the same way in 

 vertebrates of all classes, regardless of the quantity of yolk, yet the 

 presence of a large yolk mass does profoundly affect the general configura- 

 tion and topography of the layers of the embryo. Sections of embryos of 



