PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 



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 genera*! shape is that of a hen's egg 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 tJie early 

 cleavage-furrows are bilaterally arranged with respect to the plane of 

 symmetry in t/ie undivided egg ; and the same is true of the later 

 development of all the bilateral parts. 



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 egg, as has been 

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

 44, 129). In a large number of cases the egg 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 (Pcriplancta), the water-beetle (Hydrophilns\ 

 and the locust (Locus ta) that " the egg-cell possesses the- same orienta- 

 tion as the maternal organism that produces it ; it has a cephalic 

 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." 1 Wheeler ('93), after ex- 

 amining some thirty different species of insects, reached the same 



1 See Wheeler, '93, p. 67. 



