328 



IN SECT A. 



other instances, some of the most complete histories we have are due 

 to Kowalevsky (No. 416). The development of Hydrophilus has 



A 



fff 



FIG. 176. FOUK EMBRYOS OF HYDROPHILUS PICEUS VIEWED FROM THE 



VENTRAL SURFACE. (After Kowalevsky.) 

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



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 ceiitrolecithal type (vide pp. 90 99). 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 sur- 

 face of the egg, where 

 they form a thickening 

 which will be called the 

 ventral plate. At the 

 posterior part of the ven- 

 tral 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. 176 A, gg}. The 

 cells which form the 

 floor of the groove are 

 far more columnar than 

 those of other parts of 

 the blastoderm (fig. 177 

 A). The two folds on 



FlG. 177. TWO TRANSVERSE SECTIONS THROUGH 



EMBRYOS OP HYDROPHILUS PICEUS. (After Kowa- 

 levsky). 



A. Section through an embryo of the stage repre- 

 sented in fig. 176 B, at the point where the two 

 germinal folds most approximate. 



B. Section through an emhryo somewhat later 

 than the stage fig. 176 D, through the anterior region 

 where the amnion has not completely closed over the 

 embryo. 



gg. germinal groove; me. mesoblast ; am. am- 

 niuii : yk. yolk. 



