DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 



87 



Larval forms quite like that of Amphioxus have also been observed 

 in the case of Invertebrates belonging to the phyla of Ccelenterata, 

 Echinodermata, Yermes, and Brachiopoda. For the most part they 

 quit the egg-envelope, even in the gastrula stage, to swim about in 

 the water by means of their cilia ; and they can now take nutritive 

 substances small infusoria, algae, or remnants of larger animals 

 through the primitive mouth 

 into the digestive cavity, and 

 make use of them in the fur- 

 ther growth of their bodies. 

 Likewise the substances 

 which are not serviceable be- 

 cause indigestible are ejected 

 from the body through the 

 same orifice. In the case 

 of the higher animals the 

 ingestion of food is not only 

 impossible at this time, but 

 also superfluous, because the 

 egg and the embryonic cells 

 arising from it still contain 

 yolk-granules, which are 

 gradually consumed. 



The modifications which gastrulation undergoes in the Amphibia are 

 easily referable to the simpler conditions in Amphioxus. In the case 

 of the Water- Salamander, which is to serve as an illustration in 

 this description, one half of the blastula (fig. 45), which is called 

 the animal half, is thin-walled and composed of small cells, which 

 lie in two or three layers one above another, and in the case of 

 the Frog contain black pigment. The other, or vegetative half (dz\ 

 exhibits a greatly thickened wall, composed of much larger, more- 

 deutoplasmic, polygonal cells (cte), which, loosely associated in several 

 layers, cause a protuberance into the cavity (fJi) of the blastula,. 

 which is proportionally diminished in size. Where the differentiated! 

 halves meet, a transition is effected by means of cells, forming what 

 GOETTE has designated marginal zone (rz). Inasmuch as the specific: 

 gravity of the animal half is much less than that of the opposite 

 half, it is without exception directed upward in water. The former 



the outer germ-layer, as, in consequence of false observations, was formerly 

 believed, but rather from the primary inner germ-layer, as has now been esta- 

 blished by many observations. 



Fig. 45. Blastula of Triton tseniatua. 

 fh, Cleavage-cavity ; dz, yolk-cells ; rz, marginal 

 zone. 



