288 Basic Structure of Vertebrates 



neck muscles. The lateral mesoderm, remaining unsegmented, corre- 

 sponds to the hypomere of the trunk. Whereas the trunk hypomere 

 ordinarily forms only the nonstriated muscle of the digestive tube 

 and other visceral parts, the pharyngeal hypomeric mesoderm pro- 

 duces striated muscle which differentiates into an elaborate system 

 of muscles (branchiomeric muscles) related to the skeleton of the 

 jaws and gill region (Fig. 95). 



The tail is produced by growth of ectodermal and mesodermal 

 parts backward from the region of the blastopore. Growth of the meso- 

 derm keeps pace with that of the neural tube and notochord. The meso- 

 derm forms somites which produce the segmental striated caudal 

 muscle and the mesenchyme which gives rise to skeletal, vascular, and 

 connective-tissue structures of the tail. 



Relation of Yolk to Organogenesis 



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

 and notochord are necessarily much affected by the presence of the 

 bulky and inert yolk. Yet, regardless of the quantity of yolk, after the 

 germ-layers have been established the development of organs pro- 

 ceeds in vertebrates of all classes with only minor differences in details 

 of the processes. But the presence of a large mass of yolk does so pro- 

 foundly affect the general configuration of the early embryo that 

 sections of embryos of amphibian and bird at similar stages and corre- 

 sponding planes seem to be hardly comparable. 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. 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 inverte- 

 brate, 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. 



Yolk is food. The appropriate place for food is in the enteron. In 

 an amphibian embryo the yolk is contained within cells. Gastrulation 

 having established the enteron, the greater part of the embryonic food 

 is then present, not in the enteric cavity but, even better than that, 

 within the cells which constitute the wall of the enteron, where it may 

 be directly acted upon by the endodermal protoplasm and made avail- 

 able, as the blood-system develops, for transportation to all parts of 

 the growing embryo. 



The enormous yolk of the egg of a shark, reptile, or bird is morpho- 

 logically a part of the original ovum. But by the time cleavage of the 



