396 



INSECTA. 



in so many other instances, some of the most complete histories 

 we have are due to Kowalevsky (No. 416). The development 



FIG. 176. FOUR EMBRYOS OK HYDROPHILUS PICEUS VIEWED FROM THE 



VENTRAL SURFACE. (After Kowalevsky.) 

 The upper end is the anterior, gg. germinal groove; am. amnion. 



of Hydrophilus has been worked out by him more fully than 

 that of any other form, and will serve as a type for comparison 

 with other forms. 



The segmentation has not been studied, but no doubt belongs 

 to the centrolecithal type (vide pp. no 120). At its close 

 there is an uniform layer of cells enclosing a central mass of 

 yolk. These cells, in the earliest observed stage, were flat on 

 the dorsal, but columnar on part of the ventral surface of the 

 egg, where they form a thickening which will be called the ven- 

 tral plate. At the posterior part of the ventral plate two folds, 

 with a furrow between them, make their appearance. They form 

 a structure which may be spoken of as the germinal groove (fig. 



i(i) Diptera genuina (Musca, Tipula, etc.). 

 (i) aphaniptera (Putex, etc.). 

 (3) ,, pupipara (Braula, etc.). 



V. Neuroptera I ^ Neur P tera P^nipennia (Myrmeleon, etc.). 

 ( (2) ,, trichoptera (Phryganea, etc.). 



VI. Coleoptera. 

 VII. Lepidoptera. 



(1) Hymenoptera aculeata (Apis, Formica, etc.). 



(2) ,, entomophaga (Ichneumon, Platy- 

 gaster, etc). 



(3) ,, phytophaga ( Tenthredo, Sirex, etc.). 



VIII. Hymenoptera. 



