CLEAVAGE OF THE OVUM. 101 



work, excepting at one point where the protoplasm is more 

 dense. It is at this point only that the cleavage takes place ; 

 for the breaking up of the rest of the ovum into irregular 

 masses cannot be regarded as a process^ in any way related to 

 cleavage, inasmuch as the nucleus takes no part in it. 



The cleavage would appear, therefore, to be meroblastic, and, 

 as in meroblastic ova, the protoplasm round the nuclei at the 

 periphery of the blastoderm is perfectly continuous with that 

 of the main mass of the ovum in which the yolk is contained, 

 but from which it is absent in this ovum ; that is to say, we 

 have round the periphery of the blastoderm, and lying in the 

 part of it which corresponds to the yolk of large-yolked mero- 

 blastic eggs, a number of yolk-nuclei, or rather of nuclei 

 which correspond to the yolk-nuclei of such large-yolked eggs. 



But the cleavage is not only meroblastiCj it takes place in 

 the same manner as in centrolecithal ova, i. e. the furrows 

 extend only a short distance into the ovum {vide Chapter 

 II, pp. 21, 22) ; the deeper parts of the segments are con- 

 tinuous with each other. Very soon, however, the loosely 

 reticulated protoplasm extends on to the contiguous surfaces 

 of the segments, from which it was at first absent. It thus 

 happens that each segment becomes continuous with all the 

 contiguous segments near the surface, as well as in the deeper 

 parts (vide description of segmentation, p. 21, Chapter II). 

 By the continued division of the nuclei at the edge of the 

 blastoderm the embryo acquires an external layer of nuclei, 

 which are absent only at one point — the future blastopore. 



Of all the forms of Arthropodan cleavage that I know of, this 

 process seems to resemble most nearly that of the mite Tetrany- 

 chus telarius,as described by Claparede (No. 23). In this form, 

 as in Peripatus, the first segmentation nucleus divides at the 

 periphery of the ovum, and not in its centre, as in most centro- 

 lecithal ova. There is, however, the greatest possible variety 

 in the position of the first segmentation nucleus in Arthropoda 

 and the matter does not seem immediately important. The 



' This process is probably identical with the formation of the non-nucleate 

 yolk-spheres found in many Arthropoda. 



H 



