138 



EMBRYOLOGY 



If we regard one of the cross-axes as the perradius, and the other as 

 the interradius, we must, in accordance with the terminology employed 

 above (pp. 108, 115 ) for the medusae, designate as adradii those falling 

 between perradii and interradii, by means of which each quadrant is 

 halved, whereas the eight radii lying between the adradii and the cross- 

 axes should be interpolated as suhradii. The latter would nearly corre- 

 spond in position to the eight ribs, and, following the suggestion of Claus 

 (No. 4), we shall designate those lying next to the sagittal plane as suh- 

 sagittal, those lying nearer to the transverse plane as suhtramverse. 



Embryonic Development. — The embrjonic develop- 

 ment of the Ctenophora has been described principally by 

 Allman (No. 2), KowALEVSKY (No. 14), FoL (No. 7), A. 

 Agassiz (No. 1), Chun (No. 3), and Metschnikoff (No. 16). 

 It takes place in the different species in a nearly similar 

 manner. 



The Ctenophora are hermaphroditic. The generation of 

 sexual products takes place either intermittently throughout 

 the entire year, as at Naples, or it is confined to the summer 

 months, as in Northern seas (Trieste, North American coast). 

 The eggs in most cases are deposited singly and fertilized 

 in the sea- water ; however, the laying of eggs in strings of 

 about ten each has been maintained for some forms (Pleuro- 

 brachia Flem. according to Kowalevsky, Bolina according to 

 A. Agassiz). 



The eggs of the Ctenophora (Fig. 64) are enveloped by a 



delicate structureless pellicle 

 (vitelline membrane), w^hich 

 (Fig. 64 d) is rather widely 

 separated from the surface of 

 the eg^. The space thus re- 

 sulting is filled with a trans- 

 parent jelly, in which the egg 

 proper is so embedded that 

 it always lies at about the 

 middle. The structure of the 

 latter recalls the eggs of the 

 Siphonophora, Geryonidae, etc. 

 We can distinguish a super- 

 ficial layer consisting of formative yolk (ectoplasm, eh) and 



Fi6. 64. — Egg of iampetia pancerina 

 (after Chcn). ek, ectoplasm ; en, endo- 

 plasm ; d, vitelline membrane. 



