THE SEGMENTATION OF THE OVUM. 



117 



prominences, each forming a cell provided with a nucleus, the nuclei having 

 no doubt been formed by previous division in the interior of the ovum. 

 They appear at the edge of the yolk, and are separated from one another by 

 short intervals. Shortly after their appearance a second batch of similar 

 bodies appears, filling up the interspaces between the first-formed promi- 

 nences. In the fresh-water Gammarus fluviatilis the protoplasm is stated 

 first of all to collect at the centre of the ovum, where no doubt the segmenta- 

 tion nucleus divides. Subsequently cells appear at numerous points on the 

 surface, and by repeated division constitute an uniform blastoderm investing 

 the central yolk mass. This mode of formation of the blastoderm is closely 

 allied to that observed by Kowalevsky in Apis. 



Between ova with a segmentation like that of Insects, and 

 those with a segmentation like that of Penaeus, there is more 

 than one intermediate form. The Eupagurus type, with the 

 division of the first nucleus into eight nuclei before the division 



B 



C 



FIG. 53. THREE STAGES IN THE SEGMENTATION OF PHILODROMUS LIMBATUS. 



(After Hub. Luclwig.) 



of the ovum, must be regarded in this light ; but the most 

 instructive example of such a transitional type of segmentation 

 is that afforded by Spiders 1 . 



The first phenomenon which can be observed after impreg- 

 nation is the conglomeration of the yolk spheres into cylindrical 

 columns, which finally assume a radiating form diverging from 

 the centre of the egg. In the centre of the radiate figure is a 

 protoplasmic mass, probably containing a nucleus, which sends 



1 Vide Ludwig, Zeit. f. wiss. Zool., 1876. 



