BLASTULA, GASTRULA, AND GERM LAYERS 333 



more or less distinct, tissues or layers. This process is known 

 as gastrulation, and the didermic embryo itself is called a 

 gastrula. 



Among the lower forms the process of gastrulation often 

 remains a simple one, involving little more than the mere 

 rearrangement of the cells of the blastula into two nearly 

 homogeneous layers. But in the higher forms, such as the 

 Chordata, with which we are dealing, the process is greatly 

 complicated by the precocious formation of the rudiments of 

 the chief axial structures of the later developing embryo, as 

 well as by the differentiation of a third tissue, or intermediate 

 layer between the other two. The establishment of the rudi- 

 ments of the axial notochord and central nervous system, 

 characteristic structures of the Chordate embryo, is termed 

 notogenesis. These rudiments are formed out of the substance 

 of the two primary layers of the gastrula. But the formation 

 of a tissue between these converts the didermic embryo into a 

 tridermic organism. 



It is possible to analyze the whole process of development 

 during this period into these three subsidiary processes, gas- 

 trulation, notogenesis, and middle layer formation, and in 

 some instances they may occur somewhat separately and 

 successively. But usually there is much overlapping, and 

 the attempt to describe the process of gastrulation by itself 

 would give a very incomplete and incorrect view of the events 

 of this period. Consequently we shall describe all three of these 

 processes together. 



Three general types of gastrulas and modes of gastrulation 

 may be found, corresponding with the three types of blastulas 

 and ova mentioned above. Again the simplest and probably 

 the most primitive, though not the most typical, condition is 

 found in Amphioxus. On the posterior side of the blastula, in 

 the region just between animal and vegetal hemispheres, 

 Cerfontaine has described a small group of cells marked by a 

 tendency to rapid and continued multiplication (Fig. 151, A). 

 This region of active proliferation gradually extends laterally 

 around the blastula and ultimately forms a nearly complete 



