THE SEGMENTATION OF THE OVUM. 



85 



The origin and nature of meroblastic ova will best be understood 

 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, 6.) 

 This would gradually advance round the ovum and commence to 

 divide it into two halves. Before the furrow had however proceeded 

 very far it would come to the vitelline part of the ovum ; here, 

 according to the law previously enunciated, it would travel very 

 slowly, and if the amount of the food-yolk was practically infinite 

 as compared with the protoplasm, it would absolutely cease to advance. 

 A second vertical furrow would soon be formed, crossing the first 

 at right angles, and like it not advancing beyond the edge of the 

 germinal disc. (Fig. 44 B.) 



The next furrow should be an equatorial one (as a matter 

 of fact in the fowl's ovum an equatorial furrow is not formed till 

 after two more vertical furrow's have appeared). The equatorial 

 furrow would however, in accordance with the analogy of the frog, 

 not be formed at the equator, but very close to the formative pole. 

 It would therefore separate off as a distinct segment (fig. 44 C, c), 

 a small central, i.e. polar, portion of each of the imperfect segments 

 formed by the previous 

 vertical furrows. By a 

 continuation of the pro- 

 cess of segmentation, with 

 the same alternation of 

 vertical and equatorial 

 furrows as in the frog, a 

 cap or disc of small seg- 

 ments would obviously be 

 formed at the proto- 

 plasmic pole of the ovum, 

 outside which would be a 

 number of deep radiating 

 grooves (fig. 45), formed 

 by the vertical furrows, 

 the advance of which 

 round the ovum has come 

 to an end owing to the 

 too great proportion of 

 yolk spheres at the vitel- 

 line pole. 



It is clear from the 

 above that an immense ace 

 necessarily causes a partial 



FIG. 45. SURFACE VIEW OF THE GERMINAL DISC 

 OF FOWL'S EGG DURING A LATE STAGE OF THE SEG- 

 MENTATION. 



c. small central segmentation spheres; b. larger 

 segments outside these; a. large, imperfectly cir- 

 cumscribed, marginal segments; e. margin of ger- 

 minal disc. 



umulation of food-yolk at the vitelline pole 

 segmentation. It is equally clear that the 



