223 



the surface again the following spring as nearly full-grown Cteno- 

 phorse, to lay their eggs late in the summer." Fortunately the 

 eggs are so transparent that in some forms ( Pleurobrachia and 

 Bolina) the embryology can be studied, not only in the egg but 

 also through nearly all the earlier stages of the larva. 



Selecting Pleurobrachia as an example of the mode of growth, 

 we find that as in Idyia the egg consists of two layers, i. e. an 

 inner yolk mass and an outer, thin, finely granular layer surrounded 

 by a transparent envelope. The inner mass acts merely as a nu- 

 tritive mass, while the outer is the true embryonic layer, which 

 builds up the body at the expense of the central nutritive mass. 

 No nucleus nor nucleolus has been observed by Agassiz in the 

 eggs of any Ctenophorae, after they are once laid, until late in the 

 stage of segmentation. The egg divides into four and again eight 

 spheres of segmentation, each of which has, like the egg, origi- 

 nally an outer and inner mass. In a second stage of segmentation 

 small cells arise which surround the original eight large cells. 

 From these small cells the external organs are destined to arise, 

 while the larger cells form a yolk mass out of which the internal 

 organs arise. 



The embryonal layer is next formed, then the outer wall by "the 

 gradual encroachment of the actinal cells over the whole of the 

 yolk mass." Finally, the mouth (actinos- 

 tome) of the germ is formed, and after- 

 wards the digestive cavity, which results 

 from an invagination of the outer embry- 

 onic layer (ectoderm). Fig. 79 (after 

 Metschnikoff) represents the larva of a 

 Cydippe; a, primitive opening; b, gastro- 

 vascular cavity ; c, ectoderm ; d, endo- 

 derm ; e, interspace corresponding to the 

 body cavity of the larva of the polype. The development of the 

 chymiferous tubes is succeeded by that of the locomotive flappers, 

 eight or nine pairs in each row appearing before the young leave 

 the egg, and of the fringed tentacle, which attains a great length 

 after the young is hatched. 



Finally the definitive form of the Pleurobrachia is attained 

 before it leaves the egg, as seen in Fig. 80 (t, tentacles ; e, eye- 

 speck; c, c, rows of locomotive flappers; d, digestive cavity; 

 greatly magnified after A. Agassiz). 



