166 COMPARATIVE ANATOMY 



mesoblast bands and the yolk, forming the innermost germinal 

 layer or hypoblast. 



While these changes are in progress the ventral plate has 

 become pushed into the yolk in such a manner that it lies in a 

 depression surrounded on all sides by a fold of the blastoderm. 

 This fold increases in size, its edges grow together below the 

 ventral plate, and eventually meet arid fuse together. It is 

 obvious that the limbs of the fold consist of an outer and an 

 inner layer of blastoderm cells, and when they meet they fuse 

 together in such a way that the outer limbs are combined into 

 a continuous sheet of tissue, and so are the inner limbs, but all 

 connection between the two is lost. The result is that the 

 embryo is covered in ventrally by two membranes the inner 

 one, formed by the union of the inner limbs of the fold, is con- 

 tinuous with the edges of the ventral plate and forms a hood 

 known as the amnion, covering the lower side of the embryo ; 

 the outer membrane is continuous with the blastoderm which 

 elsewhere lies upon and encloses the yolk and is known as the 

 serosa. Between the amnion and serosa is a space freely open 

 to the yolk ; in some insects the yolk spherules pass into this 

 space, so that the embryo is, as it were, immersed in the yolk, 

 but this does not happen in the cockroach. The relations of 

 these embryonic membranes may easily be understood by 

 reference to fig. 41. 



To return to the formation of the embryo. We left the 

 ventral plate as a thickened patch on the lower side of the 

 blastoderm, composed of three layers epiblast, mesoblast, and 

 hypoblast, the tw<f latter layers having been formed by the 

 proliferation of cells from the floor of a groove. From what we 

 know of the development of other insects we must regard this 

 groove as the representative of the blastopore : eventually it 

 disappears, but the anus is formed by a secondary involution of 

 the epiblast at its hinder end. 



At an early period the ventral plate is divided into a series 

 of segments by transverse lines, and it should be noted that the 

 segmentation of the insect embryo resembles that of the Annelid 

 much more closely than that of the Crustacean. In Astacus, for 

 instance, the mesoblast consists at first of a number of scattered 

 cells which only at a late period of development arrange them- 

 selves into a series of transient mesoblastic pouches, and that only 

 in the abdominal region. In the cockroach the mesoblast is 



