384 CELL-DIVISION AND DEVELOPMENT 



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 results of a 

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

 and the locust {Lociista) that "the egg-cell possesses the same orien- 

 tation 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." ^ Wheeler ('93), after ex- 

 amining some thirty different species of insects, reached the same 

 result, and concluded that even when the egg approaches the 

 spherical form the symmetry still exists, though obscured. More- 

 over, according to Hallez ('86) and later writers, the egg always lies 

 in the same position in the oviduct, its cephalic end being turned 

 forwards toward the upper end of the oviduct, and hence toward 

 the head-end of the mother.^ 



2. Meaning of the Prornorphology of the Ovum 



The interpretation of the promorphology of the ovum cannot be 

 adequately treated apart from the general discussion of development 

 given in the following chapter; nevertheless it may briefly be 

 considered at this point. Two widely different interpretations of 

 the facts have been given. On the one hand, it has been sug- 

 gested by Flemming and Van Beneden,^ and urged especially by 

 Whitman,^ that the cytoplasm of the ovum possesses a definite 

 primordial organization which exists from the beginning of its exist- 

 ence even though invisible, and is revealed to observation through 

 polar differentiation, bilateral symmetry, and other obvious characters 

 in the unsegmented egg. On the other hand, it has been maintained 

 by Pflijger, Mark, Oscar Hertwig, Driesch, Watase, and the writer 

 that all the promorphological features of the ovum are of secondary 

 origin; that the egg-cytoplasm is at the beginning isotropous — i.e. 

 indifferent or homaxial — and gradually acquires its promorphological 

 features during its preembryonic history. Thus the Qgg of a bilateral 

 animal is at the beginning not actually, but only potentially, bilateral. 

 Bilaterality once established, however, it forms as it were the mould 

 in which the cleavage and other operations of development are cast. 



I believe that the evidence at our command weighs heavily on 

 the side of the second view, and that the first hypothesis fails to 



1 See \Vheeler, '93, p. 67. 



2 The micropyle usually lies at or near the anterior end, but may be at the posterior. 

 It is a very important fact that the position of the polar bodies varies, being sometimes at 

 the anterior end, sometimes on the side, either dorsal or lateral (Heider, Blochmann). 



3 See p. 298. * Cf. pp. 299, 300. 



