DISCOPHORA. 



347 



serve as type for the Leeches with jaws (Gnathobdellidae), has 

 been studied by Biitschli (No. 359). The early history of both 

 types is imperfectly known 1 . 



Formation of the layers. 



Clepsine. 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 

 animal 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 confirmation. 



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

 and the other and larger ven- 

 tral. The former I shall call 

 with Whitman the neuroblast, 

 and the latter the mesoblast. 

 The mesoblast very shortly 

 divides again. During the for- 

 mation of the neuroblast and 

 mesoblast additional epiblastic 

 small cells are added from the 

 three spheres which give rise 

 to three of the primitive epi- 

 blast cells, which may now be 

 called the vitelline spheres. 



The neuroblast next divides 

 into ten cells, of which the two 

 smaller are soon broken up 

 into epiblastic cells, while the 

 remaining eight arrange them- 

 selves in two groups of four 

 each, one group on each side 

 at the posterior border of the epiblastic cap. The two mesoblasts also take up 

 a position on the right and left sides immediately ventral to the four neuro- 

 blasts of each side. The neuroblasts and mesoblasts now commence to 



FlG. 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 blas- 

 toderm 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 Hoffmann's account (No. 36) is so different from that of other observers that 

 I have been unable to make any use of it. 



