PROMOKPHOLOGICAL RELATIOXS OF CLEAVAGE 



381 



in the egg of the tunicate Clavelma (Fig. 177), and by Watase ('91) 

 in the eggs of cephalopods (Fig. 178). In both these cases all the 

 early stages of cleavage show a beautiful bilateral symmetry, and not 

 only can the right and left halves of the segmenting Qgg be distin- 

 guished with the greatest clearness, but also the anterior and poste- 

 rior regions, and the dorsal and ventral aspects. These discoveries 

 seemed, at first, to justify the hope that a fundamental law of develop- 

 ment had been discovered, and Van Beneden was thus led, as early 

 as 1883, to express the view that the development of all bilateral 

 animals would probably be found to agree with the frog and ascidian 

 in respect to the relations of the first cleavage. 



This cleavage was soon proved to have been premature. In one 

 series of forms, not the first but the second cleavage-plane was found 



a 



r^-,;^;i;:,,., 



; ■ ':''■: 



P 



Fig. 178. — Bilateral cleavage of the squid's egg. [Watase.] 

 A. Eight-cell stage. B. The fifth cleavage in progress. The first cleavage {a-f) 

 with the future median plane; the second {l-r) is transverse. 



coincides 



to coincide with the future long axis {Nereis, and some other annelids ; 

 Crepidnla, Umbrella, and other gasteropods). In another series of 

 forms neither of the first cleavages passes through the median plane, 

 but both form an angle of about 45° to it {Clcpsinc and other leeches ; 

 RhyiicJielmis and other annelids ; Planorbis, Nassa, Un.io, and other 

 mollusks ; Discoccelis and other platodes). In a few cases the first 

 cleavage departs entirely from the rule, and is equatorial, as in Ascaris 

 and some other nematodes. The whole subject was finally thrown 

 into apparent confusion, first by the discovery of Clapp ('91), Jordan^ 

 and Eycleshymer ('94) that in some cases there seems to be no con- 

 stant relation whatever between the early cleavage-planes and the 

 adult axes, even in the same species (teleosts, urodeles) ; and even in 



