PKOMOKPHOLOGICAL RELATIONS OF CLEAVAGE 



283 



by Watase (Fig. 128). Here the form of the new-laid egg, before 

 cleavage begins, distinctly foreshadows that of the embryonic body, 

 and forms as it were a mould in which the whole development is cast. 

 Its general shape is that of a hen's Q.g% slightly flattened on one side, 

 the narrow end, according to Watase^ representing the dorsal aspect, 

 the broad end the ventral aspect, the flattened side the posterior 

 region, and the more convex side the anterior region. All the early 

 cleavage-furnnvs are bilaterally arranged zvith respect to the plane of 

 svmrnctrv in the undivided egg ; and the same is true of the later 

 development of all the bilateral parts. 



d 



P 



V 



Fig. 128. — Outline of unsegmented squid's egg, to show bilaterality. [Watase.] 



A. From right side. B. From the posterior aspect. 



a-p, antero-posterior axis ; d-v, dorso-ventral axis ; /, left side ; r, right side. 



Scarcely less striking is the case of the insect Qg2„ as has been 

 pointed out especially by Hallez, Blochmann, and Wheeler (Figs. 

 44, 129). In a large number of cases the Q.gg is elongated and 

 bilaterally symmetrical, and, according to Blochmann and Wheeler, 

 may even show a bilateral distribution of the yolk corresponding 

 with the bilaterality of the ovum. Hallez asserts as the result of a 

 study of the cockroach {Periplaneta), the water-beetle {Hydrophilns), 

 and the locust {Locnsta) that " the egg-cell possesses the same orienta- 

 tion as the maternal organism that produces it ; it has a ce])halic 

 pole and a caudal pole, a right side and a left, a dorsal aspect and a 

 ventral ; and these different aspects of the egg-cell coincide with the 

 corresponding aspects of the embryo." ^ Wheeler ('93), after ex- 

 amining some thirty different species of insects, reached the same 



^ See Wheeler, '93, p. 67. 



