Owing to the rupture of the embryonic envelopes taking place at the point where they are fused into one, the yolk does not escape in the above process, but is carried into a kind of yolk-sack, on the dorsal surface of the embryo, formed of the remains of the amnion and serous envelope. The walls of the yolk-sack either assist in forming the dorsal parietes of the body, or are more probably enclosed within the body by the growth of the dorsal parietes from the edge of the ventral plate.
Fig. 184. Three larval stages of Hydrophilus from the dorsal side, shewing the gradual closing in of the dorsal region with the formation of the peculiar dorsal organ do. (After Kowalevsky.)
do. dorsal organ; at. antennæ.
In Hydrophilus and apparently in the Phryganidæ also, there are certain remarkable peculiarities in the closure of the dorsal surface. The fullest observations on the subject have been made by Kowalevsky (No. [416]), but Dohrn (No. [408]) has with some probability thrown doubts on Kowalevsky’s interpretations. According to Dohrn the part of the serous envelope which covers the dorsal surface becomes thickened, and gives rise to a peculiar dorsal plate which is shewn in surface view in [fig. 184] A, do, and in section in [fig. 185] A, do. The ventral parts of the amnion and serous membrane have either been ruptured or have disappeared. While the dorsal plate is being formed, the mesoblast, and somewhat later the lateral parts of the epiblast of the ventral plate gradually grow towards the dorsal side and enclose the dorsal plate, the wall of which in the process appears to be folded over so as first of all to form a groove and finally a canal. The stages in this growth are shewn from the surface in [fig. 184] B and C and in section in [fig. 185] B, do. The canal is buried on the dorsal part of the yolk, but for some time remains open by a round aperture in front ([fig. 184] C). The whole structure is known as the dorsal canal. It appears to atrophy without leaving a trace. The heart when formed lies immediately dorsal to it[170].
Fig. 185. Three transverse sections through advanced embryos of Hydrophilus.
A. Section through the posterior part of the body of the same age as fig. 184 A.
B. Section through the embryo of the same age as fig. 184 C.
C. Section through a still older embryo.