S. HATTÀ. 45 



the epiblast, the chorda, the neural cord, the distal part of the mesoblat, 

 and the roof of the enteric canal. 



5) By stages, the shape of the scleromyotome is again converted 

 into an elongated triangle. The greater part of its inner long limb 

 which touches the axial organs, is transformed into the body-muscle, 

 while the outer long limb along the epiblast is flattend to form the cutis 

 layer. Finally the base of the triangle, which rests on the distal part of 

 the mesoblast and the roof of the enteric canal, is likewise thinned out 

 and evaginates, pushing its way between the muscle-layer and tbe axial 

 organs ; this is the sclerotome. 



In the dorsal region, the process of segmentation is essentially the 

 same as in the head-fold. The process goes on through the whole 

 breadth of the layer before the separation of the scleromyotome from 

 the rest of the mesoblast. Here the circumstance is more or less altered 

 by the differentiation of the Anlage of the pronephros which follows 

 immediately the segmentation. But this does not disturbe the parallel, 

 because the morphological equivalent of this section of the somite is 

 found in the head-fold, which does not develop into the pronephros, but 

 after having lost its segmental character, takes part in the formation of 

 the non-segmented lateral plates. 



The facts mentioned above agree with those in Amphioxus obtained 

 by Hatschek i) so far as concerns the stage up to the end of the meta- 

 merie segmentation and the fate of the scleromyotome. The parallel of 

 the further growth of the ventral half of the somite (the Seitenplatte of 

 Hatschek), which takes place immediately after this stage, has not 

 hitherto been demonstrated in Amphioxus. But there is little room for 

 doubt that the Seitenplatte grows ventrally (distally) after the oblit- 

 eration of the coelomic discepments and before the foundation of the 

 subintestinal vein ; for in Petromyzon, this vein appears in much later 

 stages. 



1) B. Hatschek, Studien über Enlwickelung des Amphioxus: Arb. Zoo]. Instit, 

 Wien, vol. iv, 1881. 



