188 GEORGE W. TANNREUTHER 
manifestation of simple mechanical forces. Rather are the 
blastomeres the expression of the active intrinsic forces, which 
control development from the earliest stages unto the end. 
Gravity, surface tension, cohesion and pressure no doubt are 
effective, but not to the extent that they become the control- 
ling or coordinating agents in development. The early cleav- 
ages are aS important as those occurring in later life, and may 
even be considered more so. ‘‘Also the long continued resem- 
blances which exist in the development of these different forms 
from the earliest segmentation of the eggs are as fundamental 
and deep seated as are the homologies existing in the adults.” 
The number of these resemblances in the annelids and mol- 
luses is surprisingly great. In all forms accurately studied, the 
first three generations of ectomeres give rise to the entire ecto- 
derm. The mesoblast arises at the fourth division of the pos- 
terior macromere D. The remaining members of this quar- 
tette and the macromeres produce the entoderm. The division 
and position. of the cells up to the twenty-four or thirty-cell stage 
are identical in many different species. Beyond this point 
Wilson believes a divergence between the two classes ensues, 
and that development proceeds upon two entirely different lines. 
However, subsequent investigators have shown that the supposed 
differences are more superficial, and that the points of resem- 
blances become more numerous and extend throughout longer 
periods of development. Lillie (95) showed that points of 
resemblance existed in the lamellibranchs and the annelids, 
and that in both classes there is an essential similarity between 
the development of the ‘first somatoblast.’ In annelids this 
structure develops to a greater extent than in Unio, but the two 
have many points in common. 
Mead (’97) and Conklin (97) showed that the rosette series 
had the same origin and position in ahnelids and molluscs, and 
that in both it probably gave rise to the apical sense organ. 
According to Conklin, it also gave rise to the cerebral ganglia, 
while Mead considered this particular point doubtful. Further- 
more, Mead in his annelid studies demonstrated that the same 
cells in five different annelids gave rise to the entoderm; that 
