378 CELL-DIVISION AND DEVELOPMENT 



B. Promorphological Relations of Cleavage 



The cleavage of the ovum has thus far been considered in the 

 main as a problem of cell-division. We have now to regard it in an 

 even more interesting and suggestive aspect ; namely, in its morpho- 

 logical relations to the body to which it gives rise. From what has 

 been said above it is evident that cleavage is not merely a process by 

 which the ^^g simply splits up into indifferent cells which, to use the 

 phrase of Pfliiger, have no more definite relation to the structure of 

 the adult body than have snowflakes to the avalanche to which they 

 contribute. 1 It is a remarkable fact that in a very large number of 

 cases a precise relation exists between the cleavage-products and the 

 adult parts to which they give rise ; and this relation may often be 

 traced back to the beginning of development, so that from the first 

 division onward we are able to predict the exact future of every indi- 

 vidual cell. In this regard the cleavage of the ovum often goes for- 

 ward with a wonderful clocklike precision, giving the impression of a 

 strictly ordered series in which every division plays a definite role and 

 has a fixed relation to all that precedes and follows it. 



But more than this, the apparent predetermination of the embryo 

 may often be traced still farther back to the regions of the undivided 

 and even unfertilized ovum. The Qgg, therefore, may exhibit a 

 distinct promorphology ; and the morphological aspect of cleavage 

 must be considered in relation to the promorphology of the ovum of 

 which it is an expression. 



I . PromorpJiology of the Ovum 



{a) Polarity and the Egg-axis. — It was long ago recognized by 

 von Baer ('34) that the unsegmented ^gg of the frog has a definite 

 egg-axis connecting two differentiated poles, and that the position 

 of the embryo is definitely related to it. The great embryo] ogist 

 pointed out, further, that the early cleavage-planes also are definitely 

 related to it, the first two passing through it in two meridians inter- 

 secting each other at a right angle, while the third is transverse to it, 

 and is hence equatorial.^ Remak afterward recognized the fact ^ that 

 the larger cells of the lower hemisphere represent, broadly speaking, 

 the ''vegetative layer" of von Baer, i.e. the inner germ-layer or ento- 

 blast, from which the ahmentary organs arise ; while the smaller cells 



1 ('83), p. 64. 



2 The third plane is in this case not precisely at the equator, but considerably above it, 

 forming a " parallel " cleavage. 



3 '55, p. 130. Among others who early laid stress on the importance of the egg-polarity 

 maybe mentioned Auerbach ('74), Hatschek ('77), Whitman ('78), and Van Beneden (^"^l). 



