328 INTRODUCTION TO CYTOLOGY 



the complete monopoly of the nucleus in the transmission of hereditary 

 characteristics, that the maternal character of Godlewski's larva? could 

 be explained on the assumption that the early developmental stages do 

 not require the expression of the hereditary capabilities of the nucleus, 

 but are dependent more directly upon mechanical causes. Boveri (1903, 

 1914), as a result of his hybridization experiments, strongly emphasized 

 the view that the spermatozoon has an influence upon all of the larval char- 

 acters; but he pointed out that the larval stages by themselves are not 

 sufficient grounds upon which to establish complete conclusions regarding 

 the respective roles of nucleus and cytoplasm, since the general course 

 of the early developmental stages in such organisms is immediately 

 dependent to a very large extent upon the general organization of the egg. 



This brings us to a brief consideration of the "promorphology" of 

 the highly organized animal egg, and of the relation which exists between 

 this organization and the character of the organism developing from it. 

 Of the large amount of work done in this field only a hint can be given 

 here. 



The Promorphology of the Ovum. — There arose very early two views 

 regarding the organization of the egg which recall the older theories of 

 preformation and epigenesis (Chapter I). According to one, most fully 

 expressed in W. His's Theory of Germinal Localization (1874; see Wilson 

 1900, p. 397), the embryo is prelocalized in the general cytoplasm of the 

 egg — not preformed in the old sense of Bonnet, but having its various 

 parts represented by substances with definite relative positions. This 

 view found support in those cases in which a single isolated blastomere of 

 the two-celled stage develops into a half-larva instead of a complete 

 smaller larva (Roux on the frog, 1888; Crampton on the marine gastropod, 

 Ilyanassa, 1896) ; and especially in Beroe, a ctenophore, which produces an 

 incomplete larva even if a portion of the unsegmented egg be removed 

 (Driesch and Morgan, 1895). 



Opposed to the above view was that which held the egg to be isotropic 

 and without any predetermination of embryonic parts. Certain well 

 known experiments appeared to bear out this conclusion. It was found 

 in the frog (Pfluger 1884; Roux 1885), the sea urchin (Driesch 1892), an 

 annelid (Wilson 1892), and Ascaris (Boveri 1910) that very abnormal types 

 of cleavage can be artificially induced, but that normal larvae nevertheless 

 result. A number of cases were also described in which complete embryos 

 arose from single isolated blastomeres of the two-celled stage (Fundulus, 

 Morgan 1895; and other forms), or even from those of the sixteen-celled 

 stage (Clytia, Zoja 1895). Had there been any prelocalization of parts 

 in the egg it is difficult to see how normal or complete embryos could 

 have arisen in such abnormal ways as these. 



It appears that the eggs of different animal species vary greatly 

 in the degree and fixity of their internal differentiation. In some cases 



