86 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The form of the cell and the direction of pressure cannot therefore 

 be used in explaining in any general way the direction of cleavage, as 

 proposed by Hertwig and Braem. Tlie raetliod of reaction and the 

 purpose of the reaction must be determined lor each class of cases in 

 itself. 



A consideration of the process of gastrulation leads, though from the 

 opposite direction, to conclusions of a similar nature. The form and 

 direction of cleavage are related to the later morphogenetic processes. 

 Gastrulation is a result of the method of cleavage, — and the method of 

 cleavage must be looked upon as adapted to the purpose of accomplish- 

 ing gastrulation. 



The relation of the form of cleavage to the later morphogenetic pro- 

 cesses is shown in a different way in such forms as Nereis (Wilson, '92) 

 and Unio (Lillie, '95), where it has been possible to show the exact 

 relation of later organs to individual blastomeres. Cleavage in many 

 cases is itself a direct morphogenetic process, the exact method of 

 ■which can be referred to no more simple mechanical factors than can 

 the characteristic form of the adult. 



1 do not of course wish to generalize this statement ; it is evidently 

 true for many forms, but may not be true for all. The evidence upon 

 which a contrary view is sometimes maintained for certain forms, as 

 the echinoderms, seems however inconclusive. The formation of the 

 micromeres has been shown to be preceded by a differentiation in the 

 cytoplasm (Morgan, '94), which would naturally lead to the conclusion 

 that the micromere formation is a process having a definite signification 

 for morphogenesis. But Driesch ('93) showed that the micromeres 

 might be removed and a normal larva still produced. From this, how- 

 ever, it does not follow that the formation of micromeres is without 

 definite morphogenetic significance, any more than it follows that the 

 fundaments of limbs are of no morphogenetic significance because a 

 normal larva results after the embryonic limbs have been removed from 

 a young amphibian. The formation of micromeres apparently segi-e- 

 gates a certain amount of substance which needs to be localized in a 

 definite region. If this segi-egated material is removed, there is no 

 evidence proving that similar material is not again segregated, at per- 

 haps a later stage. As Roux insists, it is important to distinguish 

 the normal method of development from induced development due to 

 injury. 



