CENTROLECITHAL SEGMENTATION. 



above nuclei with its protoplasm is present. This process, 

 known as the secondary segmentation of the yolk, is really part 

 of the true segmentation, and the bodies to which it gives rise 

 are true cells. 



Other examples of this type may be cited. In Aphis 1 Metschnikoff 

 shewed that the first segmentation nucleus divides into two, each of which 

 takes up a position in the clearer peripheral protoplasmic layer of the egg 

 (fig. 52, i and 2). Following upon further division the nuclei enveloped in a 

 continuous layer of protoplasm arrange themselves in a regular manner, and 

 form a syncytium, which becomes segmented into definite cells (fig. 52, 3 and 

 4). The existence of a special clear superficial layer of protoplasm has been 

 questioned by Brandt. 



FIG. 57. SEGMENTATION OF APHIS ROSAE. (Copied from Metschnikoff.) 

 In all the stages there is seen to be a central yolk mass surrounded by a layer of 



protoplasm. 



In this protoplasm two nuclei have appeared in i, four nuclei in 2. In 3 the nuclei 



have arranged themselves regularly, and in 4 the protoplasm has become divided into 



a number of columnar cells corresponding to the nuclei. 



TV. pole of the blastoderm which has no share in forming the embryo. 



In Tetranychus telarius, one of the mites, Claparede found on the surface 

 of the ovum a nucleus surrounded by granular protoplasm (fig. 51) ; which 

 is no doubt the first segmentation nucleus. By a series of divisions, all 

 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 2 , a layer of protoplasm is stated to appear investing the yolk ; 

 and in this there arise simultaneously (?) a number of nuclei at regular inter- 

 vals, around each of which the protoplasm separates itself to form a distinct 

 cell. Closely allied is the type observed by Kowalevsky in Apis. Develop- 

 ment here commences by the appearance of a number of protoplasmic 



1 Metschnikoff, " Embry. Stud. Insecten," Zcit. fur wiss. Zoo!., Bd. xvi. 1866. 

 My own observations on this form accord in the main with those of Metschnikoff. 



2 Vide Weismann, Entwicklung d. Dipteren; and Auerbach, Organologische 

 Studien. 



