DEVELOPMENT OF PHRYGANIDS. 571 



descended from the blastoderm ; neither is it possible as yet to 

 say how far such distinctions hold good in other forms. Judging, 

 however_, from Kowalevsky's^ figures I should think that the 

 amnion was a derivative of the ventral plate. It is the same 

 in Porthesia figured by Bobretzky,^ although the diflFerence is 

 not so well marked here as in Hydrophilus. 



We have already seen that the fusion of the amniotic folds 

 with each other produces two completely closed sacs, one within 

 the other (PI. XXXVIb, figs. 10—13). The outer sac formed 

 by the serosa, which is not fully represented in the figures, com- 

 pletely encloses the yolk. The second sac, the amnion, is also 

 completely closed, its dorsal wall being much thickened to form 

 what we shall presently describe as the " germinal band.'* 



We have seen that the ventral plate in the restricted sense in 

 which we use the term is simply an oval thickening of the blasto- 

 derm covering the ventral side of the egg. During, however, 

 the formation of the amnion and serosa this thickened area takes 

 a more definite shape, increasing in length and decreasing in 

 breadth, thus giving rise to a slipper-shaped thickening, which 

 we shall now call the '' germinal band." The head of the 

 future embryo arises from the '' heel " of this band, which at 

 this place expands into two plates, one on each side, in order to 

 form the procephalic lobes. Zaddach's statement that they 

 arise as thickenings of the " Hautblatt " is erroneous ; for, as 

 far as I am able to make out, he confounded the amnion, or 

 possibly both amnion and serosa, with the ectoderm. 



Up to this time the germinal band consists simply of a single 

 layer of similar cells; it, however, continues to increase in 

 length, until at the end of this period it extends over two thirds 

 of the circumference of the egg. 



Meantime, however, there have occurred peculiar changes 

 affecting the apparent localisation of the ventral plate and 

 germinal band. The thickening of the blastoderm occurs at one 



^ A. Kowalevsky, " Embryologische Studium an Wiirniern und Artliro- 

 poden," ' Men. Ac. imp. St. Petersbourg,' Ser. vii, vol. xvi, lS71. 



2 Bobretzky, " Ueber der Bildung des Blastoderms u. Keimblatter bei den 

 Insecteu," * Zeit. fiir wiss. Zool.,' 1S7S, vol. xxxi, pp. 195 — 215. 



