34 



BIOLOGICAL LECTURES. 



it has arisen in the course of ontogeny (oogenesis) as a typical 

 differentiation, at a typical place of the entire germ, through 

 typical formative internal stimuli " (p. 99). After such sweeping 

 concessions from the most vigorous opponent of the principle 

 of His and of the mosaic theory of Roux we may now consider 

 determinate cleavage, at least in certain cases, as no longer a 

 matter of controversy. In conclusion one may say of all deter- 

 minate cleavage that the reason that a certain blastomere arises 

 in a certain way, passes tJiroiigJi a definite developmental history, 

 and in the end gives rise to a definite part is at bottom the same 

 reason that the egg of a given animal passes tJirongh a definite 

 history and gives rise to a definite organism. 



II. Cell Homology. 



In the search for the earliest appearing homologies in the 

 development of organisms embryologists have generally been 

 content to stop with the germinal layers. This has been chiefly 

 due to the fact that there is such great diversity in the pre- 

 gastrular stages of most animals that they cannot be brought 

 into any single system. There are various types of cleavage, 

 such as the meroblastic and holoblastic, the alecithal, telole- 

 cithal, and centrolecithal, the radial, bilateral, and asymmetrical, 

 the determinate and indeterminate, and, while it is possible to 

 hypothetically connect them, it is not possible at present to 

 compare the blastomeres of one type with those of any other. 

 If any similarity ever existed between the blastomeres of an 

 arthropod and of an annelid or of a cephalopod and of a gas- 

 teropod the alteration of the type of cleavage has completely 

 destroyed it. Any attempt to establish cell homologies must be 

 limited not only to a single type, but also to determinate, that is, 

 constant and differential cleavage. In addition, any detailed 

 comparison of the cleavage stages of various animals demands 

 an accurate knowledge of the cell-origin of various parts and 

 organs, and this is unfortunately lacking except in a few cases. 



If, within the limits indicated, we compare the cleavage of 

 one species with that of other related species or genera we find 

 many identical characters running through all of them. Among 



