116 MALCOLM LAURTE. 



of the ventral plate as a single layer of flat cells to form the 

 inner embryonic membrane — the amnion proper (fig. 16^ am.). 

 This amnion never loses its connection with the epiblast as 

 the serous membrane has now done, but remains attached 

 to its edges and only extends round the egg as the epiblast 

 extends. 



The most important change in this stage is the formation of 

 the mesoblast (mes.). This layer is formed under the whole 

 ventral plate by a multiplication of the cells of the primitive 

 hypoblast, from which it is in places not yet distinguishable. 

 The mesoblast extends across the whole ventral plate from 

 side to side, and is much thicker in the somites than between 

 them. 



The serous membrane {s. m.) has, as mentioned above, now 

 lost all connection with the blastoderm, and is continued round 

 about two thirds of the e^^ by the '' peripheral cells," which 

 are now beginning to separate from the e^^ and form a definite 

 membrane. The cells of the serous membrane are becoming 

 large and flat. 



The hypoblast extends a little way beyond the ventral plate, 

 forming a single layer of cells (%.) in the periphery of the 

 yolk immediately under the serous membrane. 



By the time the embryo has reached a stage with three somites 

 completely formed (PI. XIV, fig. 17) most of the changes which 

 were going on in the last stage are completed. The amnion has 

 entirely closed over the embryo (fig. 18, am.), though its cells 

 have not yet attained their characteristic form. The mesoblast 

 {mes.) is entirely separated from the hypoblast, and remains 

 henceforth a distinct and independent layer. The hypoblast 

 {hy.) is now a single layer, extending under the whole ventral 

 plate, except in the tail-segment, where it consists of a splieri- 

 cal mass. This hypoblastic mass in the tail-segment is the 

 direct product of the primitive thickening. The hypoblast 

 extends somewhat further round the egg than the other 

 layers, as is diagrammatically shown in fig. 19. 



The description given above of the mode of formation of the 

 serous membrane and amnion difl:'ers very considerably from 



