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. The method of reaction and the 
purpose of the reaction must be determined for each class of casos 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 morphogenctic 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. 
I 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 morphogenctic significance because a 
normal larva results after the embryonic limbs have been removed from 
a young amphibian. The formation of micromeres apparently segre- 
1 
gates a certain amount of substance which needs to be localized in a 
definite region. If this segregated 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. 
