I WO I'KIM LAYERS. 



87 



Larval forms quite like tL-.i of Amphioxus i. be en observed 



in the case of Invertebrate- be!,, n-in;: to tin- phyla of Ctt'hm. 

 Kchinodermaia. VermeS, and I'.rachiop, .la. For th-mct part they 

 (jiiit the rgg-t'iiv. lope, even in the irastrula st:ir-. to swim about in 

 the water by nnans of their cilia ; and they can no\v take nunitivn 

 sub-i -mall infusoria, a I'/a-, or remnants of larger animals 



through th- primitive mouth 

 into tin- dL-v.Mive cavity, ami 

 make use of t hem in the fur- 

 ther growl h of their hodies. 

 Likewi-o the substances 

 which are not serviceable be- 

 cau>e indi:'stible are ejected 

 from tin- hotly through the 

 >ame orifice. In the case 

 of the higher animals the 

 -ion of focd is not only 

 impossible at this time, but 

 al>o superfluous, because the 

 t'lTU' and the embryonic cells 

 arising from it still contain 

 ydk-^ranules, which are 

 iriadually consumed. 



The modifications which ynntmltitioii undergoes in the A iujJii^ia are 

 easily r ferable 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 call, d 

 the animal half, is thin-walled and gomposed 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) f 

 exhibits a greatly thicken* d wall, composed of much larger, more 

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

 layers, cause a protuberance into the cavity (fh) of the blastula^ 

 \\hii-h is proportionally diminished in size. Where the differentiated 

 halves meet, a transition is ell. cted by means of cells, forming what 

 GOETTE has designated m'H-'jimd zone (rz). Inasmuch as the specific 

 gravity of the animal half is much 1- ss than that of the opposite- 

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



the outer germ-hivei, 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. 



dz 



Fig. 45. Blastula of Triton teeniatus. 

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

 zone. 



