218 Tlic Unity of the Organism 



the beginning of development, strictly understood, about 

 reaches its zcnitli among the insects. According to W. M. 

 Wheeler, a Frencli investigator, P. Hallez, states the mat- 

 ter thus with reference to the cockroach, water-beetle, and 

 locust : "The egg-cell possesses the same orientation as the 

 maternal organism that produces it : it has a cephalic pole 

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

 aspect and a ventral aspect ; and these different aspects 

 of the egg-cell coincide with the corresponding aspects of 

 the embryo." -" "My obserAations," Wheeler says, "based 

 on some thirty different insects, accord perfectly with those 

 of Hallez ;" and lie adds as details that the head end of 

 the future embryo is usually marked by the micropyle and 

 that the dorsal and ventral sides are foreshadowed by a 

 slight flexure of the elongated Qgg in its longitundinal axis, 

 the concave surface of the oigg corresponding finally to the 

 dorsal side of the embryo. To understand more specifically 

 the meaning of this for the point under consideration, it 

 is necessary to have in mind that such insect eggs as 

 Wheeler is talking about are largely yolk, so far as bulk 

 is concerned, and that the very young embryo arising from 

 division of the protoplasmic part of the Qgg occupies but a 

 relatively small portion of its whole surface. This small 

 embryonal patch or blastoderm, after it begins to elongate 

 and to show traces of the jointed body of the adult insect, 

 is called the germ-band. "The practical value of Hallez' 

 law," Wheeler says, "was shown in studying the Xiphidmm 

 [a locust] ^gg\ all the movements of the germ-band could 

 be at once referred to the axis of the mature embryo. When 

 the eggs of other insects are oriented in the same manner, 

 it is seen that the germ-band invariably arises on the ven- 

 tral surface of the yolk with its procephaleum directed to- 

 wards the cephalic, and its tail toward the caudal pole. 

 No matter what positions it may subsequently assume, it 

 always returns to its original position before hatching." ^^ 



