520 I. iKEDA : 



average. This «liows that the growth is lost in the curvutiire of 

 the body. 



AVhen the growing larva reaches the stage represented in 

 fig. 8, tlie l)last|)ore assumes a narrow transversely directed, slit- 

 like form. That portion of the larval body lying in front of the 

 hlastpore — whidi is the })ersistent larval mouth — protrudes more 

 or less prominently forwards and ventrally, so as to acquire the 

 form characteristic of the prcoral lol)c of Actinotrocha. In such 

 an advanced gastrula, the primary gut-cavity is well established 

 and can be plaiidy traced through the wall in the surface view. 

 If the larvœ of such an early stage of development l)e taken out 

 of the em])ryonal mass and set free in water, they will swim al)out 

 by means of the well developed cilia, which cover the whole 

 external surface. 



Fig. U re})resents a side view of a larva, in which the ])re- 

 oral lobe has grown to a very considerable size. The ncphridial 

 pit, which is an ect(jblastic invagination just in front of the 

 posterior end of the gut, is now distinctly visible from the out- 

 side. In short, the larva, may l»e said to possess the inceptive 

 characters of an Actinotrocha. 



I will now proceed to descril)e the internal changes accom- 

 [)anying gastrulation. The earbest symptom of this process can 

 1)0 seen in sections before it can be detected from the snrface. 

 It consists at first in a peculiar dis2)osition of those blastodermic 

 cells which constitute the ventral })ortion of the l)lastula wall. 

 This ])ortion not only shows a shallow concavity, but also the cells 

 comp()sing it become, as figs. 26 a and 2(5 h show, ii'i'egnlarly 

 arranged on account of mutual pres«urc, as a result of which some 

 of the cells are even forced out of file so as to fall into the 

 Ijlasiocœle. These liberated cells have usually a round sha[)c and 



