116 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. 



body of the embryo which, if the yolk membrane fails, has no need 

 of a special protective envelope. In this latter group, however, the 

 formation of the germinal streak, which is in close relation with 

 the formation of the nervous system, is accomplished later, during 

 the post- embryonic development, when the larva is free-swimming 

 and can procure its own food. In like manner many Polychaetes 

 and Arthropods (Branchipus) only acquire a germinal streak in 

 the course of their later growth as larvse. 



In all cases in which the embryonic development begins by the 

 formation of a germinal streak, the embryo only becomes definitely 

 limited after the yolk has been gradually surrounded, as a result 

 of processes which are connected with the complete entry of the 

 yolk into the body cavity (Frogs, Insects), or with the origin of a 

 yolk sac from which the yolk passes gradually into the body of the 

 embryo (Birds, Mammals). The progressive organization of this 

 latter, up to its exit from the egg membranes, presents in each 

 group such extraordinary variations that it is not possible to give 

 a general account of them. 



Of primary importance is the fact that in the rudiment of the 

 germ two cell layers first make their appearance one the ectoderm, 

 which gives rise to the outer integument; and the other the endoderm, 

 from which arises the lining membrane of the digestive cavity and 

 of the glands opening into ib. Between these two layers there is 

 formed, either from the outer or the inner layer, or from both layers, 

 an intermediate layer, known as the mesoderm. From the mesoderm 

 ame the muscular system and the connective tissues, the corpuscles 

 of the lymph and blood, and the vascular system. The body cavity 

 may either be derived from the persisting segmentation cavity, i.e., 

 the primitive space between the ectoderm and endoderm (primary 

 body cavity), or it may be developed secondarily as a split in the 

 mesoderm (coelom), or as outgrowths from the rudiment of the 

 alimentary canal (archenteron), in which, case it is known as an 

 enteroccele body .cavity. 



The nervous system and organs of sense are probably in all cases 

 derived from the ectoderm, very frequently as pit- or groove-like 

 imaginations which are subsequently constricted off. On the other 

 hand, the urinary and generative organs arise both from the outer 

 and inner layers as well as from the middle layer, which is itself 

 derived from one of the primary layers or from the walls of the 

 primary single -layered blastosphere. 



Accordingly, as a rule the rudiments of the skin and glandular 



