398 



INSECT A. 



general way in the same 

 manner as the amnion in, 

 for instance, the chick. 

 The folds at their origin 

 are shewn in surface view 

 in fig. 176 D, am, and in 

 section in fig. 177 B, am. 

 The folds eventually 

 meet, coalesce (fig. 178, 

 am) and give rise to two 

 membranes covering the 

 ventral plate, viz. an 

 inner one, which is con- 

 tinuous with the edge of 

 the ventral plate ; and 

 an outer, continuous with 

 the remainder of the 

 blastoderm. The verte- 

 brate nomenclature may 

 be conveniently employ- 

 ed for these membranes. 

 The inner limb of the 



A 



FIG. 178. SECTIONS THROUGH TWO EMBRYOS 

 OF HYDROPHILUS PICEUS. (After Kowalevsky.) 



A. Section through the posterior part of the 

 embryo fig. 176 D, shewing the completely closed 

 amnion and the germinal groove. 



B. Section through an older embryo in which 

 the mesoblast has grown out into a continuous 

 plate beneath the epiblast. 



gg. germinal groove ; am. amnion ; yk. yolk ; 

 cp. epiblast. 



fold will therefore be spoken of as the amnion, and the outer 

 one, including the dorsal part of the blastoderm, as the 

 serous envelope 1 . A slight consideration of the mode of 

 formation of the membranes, or an inspection of the figures 

 illustrating their formation, makes it at once clear that the yolk 

 can pass in freely between the amnion and serous envelope (vide 

 fig. 181). At the hind end of the embryo this actually takes 

 place, so that the ventral plate covered by the amnion appears to 

 become completely imbedded in the yolk : elsewhere the two 

 membranes are in contact. At first (fig. 176) the ventral plate 

 occupies but a small portion of the ventral surface of the egg, but 

 during the changes above described it extends over the whole 

 ventral surface, and even slightly on the dorsal surface both in 

 front and behind. It becomes at the same time (fig. 179) divided 



1 The reverse nomenclature to this is rather inconveniently employed by Metsch- 

 nikoflf. 



