PARTIAL SEGMENTATION. 



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 furrows 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 imper- 

 fect segments formed 

 by the previous verti- 

 cal furrows. By a 

 continuation of the 

 process of segmenta- 

 tion, with the same 

 alternation of vertical 

 and equatorial furrows 

 as in the frog, a cap or 

 disc of small segments 

 would obviously be 

 formed at the proto- 

 plasmic pole of the 

 ovum, outside which 

 would be a number of 

 deep radiating grooves 

 ( fi g- 45), formed by 

 the vertical furrows, 

 the advance of which 

 round the ovum has come to an end owing to the too great pro- 

 portion of yolk spheres at the vitelline pole. 



It is clear from the above that an immense accumulation of 

 food -yolk at the vitelline pole necessarily causes a partial seg- 

 mentation. It is equally clear that the part of meroblastic ova 

 which does not undergo segmentation is not a new addition 



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. 



