BLASTULA, GASTRULA, AND GERM LAYERS 339 



called peristomial mesoderm, to distinguish it from the gastral 

 mesoderm formed in connection with the enterocoelic grooves. 

 Very soon, as we have seen, the region which gives rise to the 

 peristomial mesoderm comes to extend nearly to the ventral side 

 of the blastopore region. The ccelomic spaces of this peristomial 

 mesoderm are not formed as derivations of the archenteron; they 

 result from rearrangements of the mesodermal cells, and are en- 

 tirely independent of the enterocoelic portions of the coelom in 

 their origin, although the two become continuous later (Fig. 

 153). These two forms of mesoderm are directly continuous 

 with one another and have indeed a common primary origin, the 

 germ ring or margin of the blastopore. If we recognize the 

 essential difference between them as that of time of formation, 

 the altered circumstances surrounding the formation of each 

 due to this time difference, become of secondary importance as 

 regards our real conception of the mesoderm and its relations 

 to the other germ layers. Thus the relation of both chorda 

 and mesoderm proper to the cells of the monodermic blastula 

 is the same as that of the endoderm proper. 



Stated in a word then, the gastrulation of Amphioxus is a 

 combination of invagination and involution, accompanied by 

 epiboly, and the processes of notogenesis and mesoderm forma- 

 tion are intimately bound up with the formation of the inner 

 layer. 



Having become familiar now with the general ideas of gastru- 

 lation and the terminology of the process we may consider the 

 remaining forms of this process in the Chordates much more 

 briefly. 



Our second type of gastrulation, as it occurs in the Amphibia 

 and Ganoid Fishes, may be easily understood by comparison 

 with the preceding. The chief differences result from the accu- 

 mulation of yolk in the vegetal pole of the ovum and blastula, 

 and the consequent comparative inertness of this region. That 

 is, the chief modifications of the typical process of gastrulation 

 appear in respect to the behavior of the lower pole, destined to 

 form the inner layer of the gastrula. 



In the Amphibian blastula, the form of which was described 



