DISCOPHORA. 



287 



Formation of the layers. 



Clepsilie. It is necessary to give a full account of the segmentation 

 of Clepsine, as the formation of the germinal layers would be otherwise 

 unintelligible. 



Segmentation commences with the division of the ovum into two 

 unequal spheres by a vertical cleavage passing from the animal to the 

 vegetative pole. By a second vertical cleavage the large segment is divided 

 into two unequal parts, and the small one into two equal parts. Of the 

 four segments so produced three are relatively small, and one, placed at the 

 posterior end, is large. Each of the four segments next gives rise to a 

 small cell at the auimal pole. These small cells form the commencement 

 of the epiblast, and, according to Whitman, the mouth is eventually placed 

 in their centre. Such a position for the mouth, at the animal pole, is 

 extremely unusual, and the statements on this head require further coii- 

 firmation. 



The posterior large segment now divides into two, one of which is 

 dorsal, and the other and larger ventral. The former I shall call with 

 Whitman the neurobiast, and the latter the mesoblast. The mesoblast 

 very shortly divides again. During the formation of the neurobiast 

 and mesoblast additional epiblastic small cells are added from the three 

 spheres which give rise to three of the primitive epiblast cells, which may 

 now be called the vitelline spheres. 



The neurobiast next divides into ten cells, of which the two smaller 

 are soon broken up into epiblastic cells, while the remaining eight arrange 

 themselves in two groups of four each, one group on each side at the posterior 

 border of the epiblastic cap. The two inesoblasts also take up a position 

 on the light and left sides im- 

 mediately venti-al to the four 

 neuroblasts of each side. The 

 neuroblasts and mesoblasts now 

 commence to proliferate at their 

 anterior border, and produce on 

 each side a thickened band of 

 cells underneath the edge of the 

 cap of epiblast cells. Each of 

 these bands is formed of a super- 

 ficial quadruple 1 row of neuro- 

 blasts budded off from the four 

 primary neuroblasts, and a deeper 

 row of mesoblasts. The com- 

 pound streaks so formed may be 

 called the germinal streaks. 



The general appearance of 

 the embryo as seen from the 

 dorsal surface, after the appear- 

 ance of the two germinal streaks, 

 may be gathered from fig. 158 A. 

 The epiblastic cap in this figure 



ni, 



pr- 



FIG. 158. Two VIEWS OF THE LARVA OF 

 CLEPSINE. (After Whitman.) 



o. oral extremity; m. mouth; pr. germinal 

 streak. 



A. This figure shews the blastoderm 

 (shaded) with a thickened edge formed by the 

 primitive (i.e. mesoblastic) streaks with the 

 four so-called neuroblasts posteriorly. The 

 vitelline spheres are left without shading. 



B represents an embryo in which the 

 blastoderm has enclosed the yolk, and in 

 which the division into segments has taken 

 place. At the hind end are shewn the so- 

 called neuroblasts forming the termination of 

 the germinal streak. 



1 According to Eobin it is more usual for there to be only a triple row of primary 

 neuroblasts. 



