252 BIOLOGICAL LECTURES. 



might be cited, but these, I think, are amply sufficient for the 

 present purpose. 



The facts given above appear to afford conclusive evidence 

 in support of the view that there has been an increasingly early 

 appearance of morphogenetic cleavage and considerable inde- 

 pendent modification in the phylogenetic history of the annelids 

 and mollusks and their ancestors. Nevertheless, it must be 

 admitted that there is a high degree of resemblance in the 

 cleavage even of widely separated forms. The tendency has 

 been to emphasize these similarities, even to the extent of 

 neglecting whatever differences might be found. Comparison 

 of details shows that many and considerable differences do 

 exist, and it shows further that the resemblances that occur 

 concern the more general features of the development. Two 

 facts seem to me to point the way to an explanation of these 

 similarities. First, there is in all cases the absolutely identical 

 (or reversed) spiral form of cleavage in the earlier stages to 

 serve as a basis for later modifications. And, second, the larval 

 form, when present, is similar in the different groups. Where 

 the larval form is not present, as in the oligochaetes and leeches, 

 the cleavage departs at an early stage from the plan adhered 

 to in the other groups. Whether the trochophore be regarded 

 as palingenetic or ccenogenetic, its similarity of form in the 

 various groups must have been closely connected with the pre- 

 cocious appearance of resemblances in earlier stages of develop- 

 ment. Conklin remarks : " The ' reflection ' of similar larval or 

 adult characters would produce similar effects upon different 

 eggs, and consequently the similarity of the prelarval stages of 

 annelids and mollusks may be held to be due to the similarity of 

 their larvce ; but there is no reason for supposing that this par- 

 allel precocity has been independently acquired by annelids and 

 mollusks, since it may well have been produced before the phy- 

 logenetic separation of these groups." Conklin has not recog- 

 nized the influence of the strictly spiral form of cleavage of the 

 early stages in producing similar later stages, probably because 

 he regards the whole cleavage from the beginning as morpho- 

 genetic. I agree with him in regarding it as quite possible 

 that the " reflection " or most of it occurred before the separa- 



