120 MALCOLM LAURIE. 



(2) These multiply and form a thick mass (Stages B^ C). 



Second Period. 



(3) The serous membrane grows up from the edges of the 

 blastoderm over its surface as a single layer of cells, and is 

 continued round the yolk by the peripheral cells (Stages D — F). 



(4) The hypo-mesoblast is formed partly as a single layer of 

 cells split off from the under surface of the blastoderm and 

 partly, at the tail end, as a thick mass, the primitive thickening, 

 which probably represents an invagination. Before and up 

 to this stage cells pass from the blastoderm into the yolk 

 (Stage D). 



(5) The mesohlast is formed as a layer several cells thick, 

 extending right across the blastoderm. The hypoblast remains, 

 after the formation of the mesoblast, as a single layer, except 

 in the region of the primitive thickening, where it is a spherical 

 mass (Stage E). 



(6) The amnion is formed as a single layer of cells growing 

 up from the edges of the epiblast, with which it retains its 

 connection. The serous membrane has by this time lost all 

 connection with the blastoderm, and spreads round the greater 

 part of the yolk (Stage F). 



The embryo by this time consists of three somites and the 

 large head- and tail-segments. The somites are formed from 

 the tail in regular succession. 



Third Period. 



(7) The mesoblast divides into two longitudinal bands, and 

 ccelomic spaces are formed in the somites and in the head 

 (Stage G). 



(8) The epiblast and amnion begin to spread round the egg 

 beyond the limits of the ventral plate (Stage G). 



(9) The neural groove is formed by a thinning of the epiblast 

 in the middle line (Stage H). 



(10) The epiblast in the head-segment begins to thicken to 

 form the cerebral nervous system (Stage H). 



