THE SEGMENTATION OF THE OVUM. 



103 



Partial segmentation. The next type of segmentation we 

 have to deal with has long been recognized as partial segmenta- 

 tion. It is a type in which only part of the ovum, called the 

 germinal disc, undergoes segmentation, the remainder usually 

 forming an appendage of the embryo known as the yolk-sack. 

 Ova belonging to the two groups already dealt with are fre- 

 quently classed together as holoblastic ova, in opposition to ova 

 of the present group in which the segmentation is only partial, 

 and which are therefore called meroblastic. For embryological 



A 



FIG. 44. SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION IN A 



FOWL'S EGG. (After Coste.) 



a. edge of germinal disc. b. vertical furrow, c. small central segment, d. larger 



peripheral segment. 



purposes this is in many ways a very convenient classification, 

 but ova belonging to the present group are in reality separated 

 by no sharp line from those belonging to the group just 

 described. 



The origin and nature of meroblastic ova will best be under- 

 stood by taking an ovum with an unequal segmentation, such as 

 that of the frog, and considering what would take place in 

 accordance with the laws already laid down, supposing the 

 amount of food-yolk at the vitelline pole to be enormously 

 increased. What would happen may be conveniently illustrated 

 by fig. 44, representing the segmentation of a fowl's egg. There 

 would first obviously appear a vertical furrow at the formative 

 or protoplasmic pole. (Fig. 44 A, />.) This would gradually' 

 advance round the ovum and commence to divide it into two 

 halves. Before the furrow had however proceeded very far it 



