124 whitman: [;\^ol. I. 



of the macromeres. The earliest entoderm cells are formed in 

 precisely the same manner in Clepsine ; but here a few cells are 

 left to multiply in the macromeres, and these take the form of 

 " entoplasts," and are to be regarded as the equivalents of the 

 large mesenteric cells of Nephelis. The oesophagus of Nephelis 

 is probably lined with cells derived from the first-formed entoderm 

 cells, precisely as is the case in Clepsine. That tJiere is no 

 fundamental distinction between the " ejitoplasts " of Clepsine ajtd 

 the entoderm cells of Nephelis is shown by the fact that we have 

 both modes of formation in Clepsine, the one passing gradually 

 and insensibly into the other. 



Bergh (No. 9, p. 260) explains the above-named difference 

 in the position of the vitelline macromeres as the result of a 

 retardation in the development of the mesenteron in Clepsine, 

 and apparently regards the Nephelis type of development as the 

 more primitive. The view presented above appears to me more 

 satisfactory. I am not by any means ready to adopt the idea 

 that the mode of development in Clepsine is to be regarded as 

 a modified or derived form of that seen in Nephelis. It would 

 be quite as rational to take just the opposite ground, and main- 

 tain that the t^'g of Nephelis has been derived from one that 

 was heavily loaded with food-yolk. The mode of cleavage, and 

 especially the persistence of the three macromeres, appears to 

 support such a view.' 



Rhynchelmis (Euaxes). — In spite of the many gaps in our 

 knowledge of the development of Nephelis, it has been easy to 

 find a close and interesting parallel between it and Clepsine in the 

 early phases of the egg. This, to be sure, is no more than 

 might have been expected in the case of two forms so closely 

 allied, but it is more than could have been conceded without 

 first showing how differences of opinion could be reconciled. If 

 now we extend the comparison along the same lines to one of 

 the Chstopods, ^.^., Rhynchelmis, we shall find the suggestions 

 advanced in the foregoing pages corroboi-ated in many points of 

 fundamental importance. 



' The power to elaborate and store nutritive yolk comes and goes with the need ; 

 but the mode of development induced by the presence of yolk appears to persist, to a 

 greater or less extent, even after the yolk has been lost. Such has, in all probability, 

 been the case with the mammalian egg, and there is reason to suspect that other 

 alecithal eggs have had a similar history. Many anomalies of development may be 

 accounted for in this way. 



