IV. i INITIAL STRUCTURE OF THE GERM 



209 



the second furrow. This becomes the transverse, while the first 

 furrow marks the sagittal plane of the future organism. Each 

 of these cells contains both granular protoplasm and yolk, but at 

 the succeeding division nearly all of the former material passes 

 into the eight small cells, which are nipped off and lie in an oval 

 ring on the vegetative side of the egg 1 (Fig. 118). These small 

 cells or micromeres will give rise to the ectoderm, the large cells 

 or macromeres to the endoderm and mesoderm, the mesoderm 



FIG. 118. Normal segmentation of the Ctenopliore egg. a a, Stomach 

 or sagittal plane ; bb, funnel or transverse plane. C, E, from micromere 

 pole ; D, F, from tlie side. (After Ziegler, from Korschelt and Heider.) 



being separated off later on at the vegetative pole, which becomes 

 the oral pole of the embryo. In normal development, therefore, 

 the embryonic axes are marked out at quite an early stage. 

 Chun was the first to isolate the \ and |- blastomeres of 



1 Ziegler's description is that in this (the fourth) division the furrow 

 first appears near the animal pole, but yolk then streams from the 

 larger vegetative into the smaller animal portion, until the latter becomes 

 the inacromere, the former the micromere. Division is then completed 

 and the micromeres are at the vegetative pole. This explains the 

 divergences in the accounts of earlier observers (Agassiz, Kowalewsky, 

 Chun, Fol, and Metschmkoff;. 



JENK1NSON 



