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 tlie 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, f^^ff^^ 



and the latter the mesoblast. )\ V\ 



The mesoblast very shortly ^^^ < 



divides again. During the for- Hy 



mation of the neuroblast and V^^SsSs*^ 7 



mesoblast additional epiblastic 



/jf 

 small cells are added from the 



., , , , FlG. 158. TWO VIEWS OF THE LARVA OF 



three spheres which give rise CLEPSINE 5 (After whitman .) 



to three of the primitive epi- . . 



o. oral extremity ; m mouth ; fr. germinal 

 blast cells, which may now be streak, 

 called the vitelline spheres. A. This figure shews the blastoderm (shaded) 



The neuroblast next divides th * th j < j ke d f d ^ form ^ ed ^ , the P ri f|ve 



(t.e. mesoblastic) streaks with the four so-called 



into ten cells, of which the two neuroblasts posteriorly. The vitelline spheres 

 smaller are soon broken up are left without shading. 



into epiblastic cells, while the 



toderm has enclosec _, , 



remaining eight arrange them- division into segments has taken place. At the 

 selves in two groups of four hind end are shewn the so-called neuroblasts 

 each, one group on each side formin & the termination of the g ern ' inal streak - 

 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 



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. 



