78 CHOKDATE ANATOMY 



the unstriated muscle of the digestive tube and other visceral parts, the 

 pharyngeal h>^omeric mesoderm produces striated muscle which dififeren- 

 tiates into an elaborate system of muscles (branchiomeric muscles) 

 related to the skeleton of the jaws and gill region (Fig. 189). 



The tail is produced by growth of ectodermal and mesodermal parts 

 backward from the region of the blastopore. Growth of the mesoderm 

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

 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 

 the notochord are necessarily much affected by the presence of the bulky 

 and inert yolk. 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. Yet at early stages of development the embryonic material 

 may not be so rigidly determined. By appropriate operations at suffi- 

 ciently 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. 



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 available, 

 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 mor- 

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

 the germ-disc has progressed so far as to produce a many-celled blastoderm 

 spreading out thin and flat on the surface of the yolk, the cells of the 

 blastoderm can be regarded as, at most, merely joint proprietors of the 

 food supply and the yolk is essentially e.xtra-cellular. As development 

 proceeds, the blastoderm differentiates into the typical germ layers, the 

 mesoderm splits to form somatic and visceral sheets with coelomic space 

 between them, and all -of these layers progressively spread over and around 

 the non-living yolk until eventually it is entirely enclosed (Fig. 74) by 



