9fi 



CENTROLECITHA L HKG MENTA T1ON. 



on the surface, a layer of cells becomes formed round a central yolk mass. 

 The result here is the same as in Insects, but the nucleus with its granular 

 protoplasm is from the first superficial. In. other cases, such as that 

 of the common fly ', a layer of protoplasm is stated to appear investing the 

 yolk; and in this there arise simultaneously (?) a number of nuclei at 

 regular intervals, around each, of which the protoplasm separates itself to 

 form a distinct cell. Closely allied is the type observed by Kowalevsky 

 in Apis. Development here commences by the appearance of a number of 

 protoplasmic 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 

 prominences. 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 

 segmentation 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 Peneus, there is more than one 

 intermediate form. The Eupagurus type, with the division of the first 

 nucleus into eight nuclei before the division of the ovum, must be 



B 



c 



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



(After Hub. Luchvig. ) 



regarded in this light ; but the most instructive example of such a 

 transitional type of segmentation is that afforded by Spiders ? . 



The first phenomenon which can be observed after impregnation 

 is the conglomeration of the yolk spheres into cylindrical columns, 



1 Vide Weismami, Entirivkiinui d. Diptcnn : aiul Auerbach, Organologische 

 Stvdien. 



3 Vidf Lndwi},', Zeit. f. wi-w. Zoo!., 1876. 



