270 ELEMENTS OF BIOLOGY 



cap of dividing cells. The influence of the amount and distribution 

 of the yolk on cleavage and gastrulation is shown in the accompany- 

 ing figure (Fig. 183). 



Mesoderm Formation. Cleavage and gastrulation are fol- 

 lowed by the rise of the intermediate layer, the mesoderm. In verte- 

 brates the chief source of the mesoderm is from cells of the vicinity 

 of the lips of the blastopore, particularly the dorsal lip (Fig. 179) 

 It extends forward and laterally between the two primary layers. 

 Its lateral sheet on either side the mid-dorsal line splits into two 

 layers; the inner becomes closely applied to the endoderm, the 

 outer lines the ectoderm. The cavity set up between the two layers 

 of mesoderm is the coelom (Figs. 81 and 179). Meanwhile the 

 embryo has been elongating, the blastopore becoming smaller and 

 forming an aperture in the posterior end of what is now a more or 

 less cylindrically shaped body. These processes of germ-layer forma- 

 tion and accompanying changes in shape are modified in many 

 forms but the essential features can always be recognized. 



At the close of the period of gastrulation and mesoderm forma- 

 tion the embryo presents the basic structures of the adult organ 

 systems laid down in their permanent relation in external, inter- 

 mediate, and internal layers. Shifts of position occur in later de- 

 velopment but do not involve any change in the relative relations 

 of ectoderm, mesoderm, and endoderm. Immediately following the 

 formation of the three germ layers as described, the embryos of 

 most invertebrates develop rapidly into free swimming or moving 

 larvae of various sorts. Many types of invertebrate larvae carry on 

 independent feeding and some become sexually mature and repro- 

 duce a new generation before they themselves have become adult. 

 Some invertebrates (for example, Annelida) pass through several 

 larval stages. 



All of the sub-phyla of the chordates except the vertebrates pass 

 through a larval stage that is free swimming and somewhat similar 

 in shape to the frog tadpole. In the sub-phylum' Vertebrata, how- 



