No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 69 



vertebrates, or even as compared with the Crustacea, which 

 usually have much thinner envelopes than insect eggs. Special 

 provision is also made in many of the Crustacea for aerating the 

 eggs. Now the cells of the rapidly growing insect embryo not 

 only absorb and metabolize the yolk but also give off a certain 

 amount of waste matter. That this is not wholly of a gaseous 

 nature is seen in older embryos which have considerable 

 accumulations of uric salts in the blood corpuscles and fat- 

 body. Waste products are undoubtedly given off during the 

 stages preceding anatrepsis, and probably permeate the yolk in 

 the immediate neighborhood of the germ-band. As the oxida- 

 tion of these waste products is very probably retarded by slug- 

 gish transpiration, and as growth under such conditions would 

 be seriously impeded, we may suppose that the embryo has 

 acquired the habit of moving to another part of the Qgg where 

 the yolk is as yet unpolluted. Here it grows apace till the 

 surrounding yolk is again charged with excreta. Growth is 

 then temporarily suspended and the embryo moves back to the 

 ventral surface. The embryo reaches a considerable size be- 

 fore katatrepsis, so that its rotation must cause a considerable 

 circulation in the yolk bodies. This would also serve to aerate 

 the yolk and to bring fresh pabulum in contact with the 

 assimilating cells of the embryo. It may also be noted that 

 in many insects the movements set in at critical periods of 

 growth. Thus in XipJiidmin anatrepsis occurs during the 

 addition of new segments, and in many other forms it im- 

 mediately precedes the formation of new segments. In the 

 Orthoptera, katatrepsis usually occurs in the spring and is the 

 signal for a decisive advance in the development of the heart, 

 sexual organs, compound eyes, etc. During this period, also, 

 the abortion of such rudimental structures as the pleuropodia, 

 abdominal appendages and envelopes seems to be hastened. In 

 short, the whole process of katatrepsis, at least in XipJiidium, 

 has the aspect of rejuvenescence. It will be remembered that 

 the amnion is formed just before or during anatrepsis. It is 

 probable that the complete abstriction of this envelope from 

 the serosa is a device for favoring the movements of the embryo. 

 The germ-band is thereby set adrift on the yolk and enabled to 



