248 ZOOLOGY. 



shell-membrane and shell. The structure of an ordinary 

 hen's egg is shown in fig. 125. The ovum (" yolk ") is held 

 in place by twisted strands of firmer albumen the chalazce. 

 The shell -membrane divides at the broad end into two 

 layers, between which is an air-space. Owing to its lighter 

 character, the germinal area is always on the top of the 

 yolk, however much the egg may be turned round the 

 ovum (" yolk ") swinging on the chalazse. 



2. Meroblastic Segmentation. The first segmentation- 

 nucleus, of course, lies in the germinal area, as did the 

 germinal vesicle to start with. Segmentation begins as usual 

 by division of the nucleus into two and four, and meridional 



Fig. 125. DIAGRAM OF FOWL'S Eoo AT TIME OK LAYING. 



planes of division appear in the protoplasm of the germinal 

 area. But the whole ovum is not divided into two, four, etc., 

 cells; the meridional planes die off as they pass into the main 

 yolk-laden mass. For some time only meridional divisional 

 planes are formed, but presently (after a corresponding 

 division of the nuclei) a horizontal divisional plane appears 

 in the protoplasm of the germinal area, and cuts off an upper 

 layer of complete cells from the main mass of the ovum, 

 which contains, immediately under this upper layer, a 

 " lower layer " of nuclei, surrounded by imperfectly defined 

 cells which merge downwards into the undivided yolk- 

 laden mass below. Between the two layers a narrow space 

 appears the segmentation -cavity. Obviously we have 

 here a modified blastosphere stage : the upper layer answers 

 to the frog's primitive epiblast, and the rest of the ovum 



