CHAPTER VI. 



GERM LAYERS.* 



THE TWO PRIMARY GERM LAYERS FORMATION OF THE GASTRULA. 

 Gastrulation in Amphioxus. 



The changes which immediately follow the formation of the blastula can be 

 observed in their simplest form in Amphioxus, where, it will be remembered, 

 the blastula is a hollow sphere the wall of which consists of a single layer of cells 

 which enclose the segmentation cavity (Fig. 26,6). Gastrulation begins by a 

 flattening of the ventral wall of the blastula (Fig. 34, A). This is followed by 

 a folding in or invagination of the yolk cells which form the ventral wall (Fig. 

 34, B). These cells press upward into the segmentation cavity which they soon 

 completely obliterate, and come to lie immediately beneath and in contact with 

 the smaller cells which had formed the roof of the cavity (Fig. 34, C). 



The gastrula, as the embryo is now called, thus consists of two layers of cells 

 which lie in close apposition and enclose the new cavity, the archenteron (ccelen- 

 teron primitive gut) formed by the invagination (Fig. 34, C and >). This 

 cavity remains open externally, the opening being known as the blastopore 

 (Fig. 34, C and Z>). These two layers of cells which form the wall of the gastrula 

 are the primary germ layers. The outer layer is known as the ectoderm or 

 epiblast, the inner layer as the entoderm or hypoblast. As seen by reference to 

 Fig. 34, C and D, the two primary germ layers are directly continuous with each 

 other at the blastopore. 



The most significant feature of the transformation of the blastula into the 

 gastrula is that whereas in the blastula all the cells are essentially similar, 

 differing if at all only in the amount of yolk contained, in the gastrula two dis- 

 tinct types of cells are recognizable. The cells of the outer layer differ from 

 those of the inner layer both structurally and functionally. Thus in some of the 

 lowest forms the gastrula stage is the adult stage. In such the outer cells are 

 protective, react to external stimuli, develop cilia which determine locomotion, 

 etc. The inner cells, on the other hand, are more especially concerned with 

 nutrition, absorbing food, and giving off waste products. Von Baer's apprecia- 



*For many of the ideas contained in this chapter, especially the correlation of gastrulation and 

 the formation of the mesoderm in different forms, the writers are indebted to Bonnet's excellent de- 

 scription in his " Lehrbuch der Entwickelungsgeschichte." 



The homologizing of gastrulation in the different forms has been found the most satisfactory 

 method of teaching the subject. At the same time it must be admitted that some of the correlations 

 are not based on actual observations. 



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