242 BIOLOGICAL LECTURES. 



of phenomena which, as I believe, can be satisfactorily inter- 

 preted from the organism standpoint alone. 



And first I am inclined to believe that the flattening of the 

 first two blastomeres and of later ones against each other, a 

 phenomenon of very wide occurrence {cf. Figs. 2 and 3), 

 may be the result of material connections between the cells, 

 which bring the cells into contact with each other over the 

 largest possible portion of their surface after the intracel- 

 lular pressure accompanying mitosis has begun to diminish. 



In many cases the reversal of the direction of spiral cleav- 

 ages, especially when occurring in one or two quadrants, is 

 indicative of interrelation; for, as Conklin states ('97, p. 186), 

 the ultimate cause of reversal is, in most cases, the precocious 

 appearance of certain organs or planes of symmetry. 



The relative time of differentiation of various organs, and 

 especially of the early larval organs, such as the prototroch, 

 affords to my mind a most striking example of the interrelation 

 of all parts of the developing Qgg. For instance, in AmpJiitrite 

 and Lepidonotiis the primary trochoblasts become ciliated before 

 the sixty-four-cell stage (Mead, '97). In Nereis the cilia appear 

 at about the tenth hour, at a stage when the closure of the 

 blastopore is nearly completed. In Arenicola the first traces 

 of ciliation appear when the embryo is from seventeen to nine- 

 teen hours old, after the blastopore has closed completely and 

 the embryo consists of hundreds of cells. In Crepidula the 

 trochoblasts are very small and remain apparently quiescent 

 up to a late stage, then increasing in size, and still later 

 attaining their definitive functional condition. In IscJinocJiiton 

 (Heath, '99) the velum becomes ciliated much earlier than in 

 Crepidula, — thirty-one hours, — and there is no marked in- 

 crease in size of the cells. Now in all these cases the so-called 

 primary trochoblasts arose in a very similar manner and at 

 essentially the same time in the history of the ^gg, yet for 

 some reason the time of formation appears not to be related 

 directly to the time at which they become functional. 



It is important to note that in each case the differentiation 

 occurs at such a time that the trochoblasts shall be prepared 

 to perform their function when called upon by the environment. 



