40 BIOLOGICAL LECTURES. 



comparison would bring these two groups into still closer 

 agreement. (4) Finally, experiment has shown that the form of 

 determinate cleavage, which alone is under consideration, may be 

 modified in certain regards without materially modifying the re- 

 sults of development. It must not be supposed, however, that 

 such experiments destroy belief in either determinate cleavage or 

 cell homology. That certain forms of cleavage are determinate, 

 i.e., under normal or usual conditions constant and differential, 

 is a visible fact; that certain cells in related animals normally 

 give rise to the same parts is also a fact which cannot be 

 denied. Experiment shows that this normal condition may be 

 modified ; it does not prove its non-existence. Even if it 

 should be shown that the apical organ might be formed in the 

 absence of the apical cells or that the mesoblast might appear 

 after the removal of the cell 4d and be it observed such a 

 thing has never been proved the case would not be funda- 

 mentally different from the regeneration of adult parts after 

 their complete loss, and the doctrine of homology would no 

 more be destroyed in the one case than in the other. On the 

 whole, experiments on determinate cleavage (e.g :, Driesch and 

 Morgan 1 on the ctenophore and Crampton 2 on the gasteropod) 

 lend support to the doctrine of cell homology. 



A consideration of these difficulties, especially of the first and 

 second, shows how futile is any attempt to establish the uni- 

 versal homology of blastomeres, and it indicates, as Wilson has 

 pointed out in his lecture on the "Embryological Criterion of 

 Homology," that embryological likeness or unlikeness is not in 

 itself a sufficient test of homology; it indicates, as do many 

 other considerations, that the early stages of development have 

 undergone profound modifications in the course of evolution, 

 but it does not prove that these early stages never resembled 

 each other or that no traces of such primitive resemblance can 

 now be found between related organisms. In all respects the 

 same objections as those presented above may be urged against 

 the homology of many embryonic structures and processes. 



1 Op. cit , p. 24. 



2 Crampton, H. E., " Experimental Studies on Gasteropod Development," Arch, 

 fur Entwicklungsmechanik, Bd. 3, 1896. 



