I06 BIOLOGICAL LECTURES. 



larger part, if not the whole of the musculature of the trocho- 

 phore, comes from this larval mesoderm, and that it is only 

 as the larva elongates that the definitive mesoderm takes any- 

 important part in body formation. 



For this comparison I have used AmpJiitrite as being the most 

 convenient for the purpose, but exactly similar results would 

 have been attained had I used any of the other annelids with 

 unequal cleavage, whose cell lineage is at all accurately known. 

 Of the annelids with equal cleavage, Podai'ke happens to be the 

 only one which has yet been studied with any completeness ; 

 but, so far as one can judge from the figures given by Hatschek^ 

 for Eitpomatiis, Van Drasche^ for Pomatoceros, and Mead for 

 Lepido7iotiis, similar results would be obtained there, were the 

 development fully studied. The trochophores of Eupomatns 

 and Pomatoceros are thin walled, and with exactly the feeble 

 development of ectodermal and mesodermal tissue that has been 

 noted for Podarke. The same is true of Hydroides, as I am 

 informed by Dr. Wilson, and of Lepidonotiis, according to Dr. 

 Mead. Connected with the absence of a large amount of yolk in 

 these forms is the rapid development of the archenteron, so that 

 the embryo begins to feed much earlier than in the other case. 



How now are we to explain the difference in manner of 

 cleavage which has been observed in these forms .-* Why does 

 one Q.gg divide with absolute equality, while others, little or no 

 larger, divide very unequally } Within certain limits, undoubt- 

 edly, presence or absence of yolk does affect the form of 

 cleavage. It can hardly be disputed that the peculiar form 

 of cleavage found, for example, in the ^gg of the teleost is 

 caused by an accumulation of yolk at the lower pole, or that a 

 similar cause must be invoked to explain the form of segmenta- 

 tion of the egg of the bird ; and earlier writers assumed almost 

 universally that to the effects of yolk, either actually present, 

 or exerting its influence through the action of heredity, must 

 be ascribed the differences in the manner of cleavage. Rabl ('79), 

 in his work on Planoi'bis,^ finds a series of molluscs in which, 



1 " Entwick. von Eupomatus uncinatus," Arbeiten a. d. Inst, zii Wien, 1885. 



- Beitrage zu Entw. d. Polychaeten. Wien, 18S4. 



3 "Entwick. der Tellerschnecke," Morph. Jahrb., 1879. 



