246 



INVEETEBEATA 



CHAP. 



96 per cent alcohol a treatment which resulted in the embryonic 

 area being coloured dark blue, while the rest of the egg was nearly 

 colourless. Such eggs were then used for sections. 



When it was desired to have whole mounts of the embryonic 

 area, the chorion was carefully removed from the individual egg by 

 means of a fine needle, the eggs were then stained for twenty-four 

 hours in borax carmine, and differentiated for the same period in 

 acid alcohol. 



For cutting sections the fragments of the cocoon, in which all the 

 contained eggs were parallel to one another, with the future head ends 

 pointing in the same direction, were passed through xylol into paraffin. 

 Wheeler mentions that when he used paraffin melting at 55, the yolky 

 contents of the egg took on a gummy consistency which rendered it 

 specially suitable for cutting, and that he got perfect sections. This 

 must be regarded as a somewhat exceptional circumstance, because 

 yolk is usually apt to become very brittle on heating, and to break up 

 into small fragments under the stroke of the knife, hence, usually, in 

 dealing with yolky eggs, preliminary embedding in celloidin, as 

 described in Chap. II., is desirable. 



The egg of Donacia, like that of most insects, is of an elongated 

 oval form, and the nucleus is situated near the centre, surrounded 



by an island of cytoplasm. 

 When the egg is fertilized 

 the zygote nucleus begins 

 to divide and gives rise to 

 many nuclei, each sur- 

 cui. rounded by its cytoplasmic 

 island. In Dorypliora, 

 Wheeler got every stage 

 from the first and was able 

 to observe that the divisions 

 of the daughter nuclei are 

 at first strictly synchronous, 

 so that in a given egg all 

 will be in the same phase 

 of karyokinesis. When a 

 been formed some of them 



n 



FIG. 193. Portion of a sagittal section through the 

 developing egg of Doryphora (Leptinotarsa) 

 decemlineata before the formation of the blasto- 

 derm. (After Wheeler.) 



cyt, peripheral layer of cytoplasm ; n, nuclei in islands 

 of cytoplasm ; y, yolk spheres. 



considerable number of nuclei have 



begin to wander outwards towards the surface of the egg whilst 

 others remain in the interior. This wandering seems to be due to 

 amoeboid movements on the part of the cytoplasmic islands which 

 surround the nuclei, for these are often drawn out into comet-like 

 shapes. 



When the nuclei reach the surface they increase by further 

 division, and eventually form a blastoderm consisting of a layer of 

 columnar cells covering the whole ventral surface, and a flattened 

 epithelium on the dorsal surface. Towards the hinder end of the egg, 

 on the ventral surface, the columnar cells form a small mass several 

 cells deep ; this mass corresponds to the secondary thickening or 



