PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 



28l 



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 wguld probably be found to agree with the frog and ascidian 

 in respect to the relations of the first cleavage. 



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

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



C 



Fig. 126. Bilateral cleavage of the tunicate egg. 



A. Four-celled stage of Clavelina, viewed from the ventral side. B. Sixteen-cell stage (VAN 

 BENEDEN and JULIN). C. Cross-section through the gastrula stage (CASTLE); a, anterior; 

 p, posterior end ; /, left, r, right side. [Orientation according to CASTLE.] 



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

 Crepidida, 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 (Clepsinc and other leeches ; 

 RhyncJichnis and other annelids ; Planorbis, Nassa, Unio, 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 



