There are some grounds for thinking that the cases of partial segmentation in the Arthropoda are not really quite comparable with those in other groups, but more probably fall under the next type of segmentation to be described. The grounds for this view are mentioned in connection with the next type. In most if not all meroblastic ova there appear during and after segmentation a number of nuclei in the yolk adjoining the blastoderm, around which cells become differentiated. (Figs. 46 and 47.) These cells join the part of the blastoderm formed by the normal segmentation of the germinal disc. Such nuclei are formed in all craniate meroblastic ova[52]. In Cephalopods they have been found by Lankester, and in Oniscus by Bobretzky. They have been by some authors supposed to originate from the nuclei of the blastoderm, and by others spontaneously in the yolk. [52] Though less obvious in the ovum of the fowl than in that of some other types, they may nevertheless be demonstrated there without very much difficulty. Some of the earliest observations on these nuclei were made by Lankester[53] in the Cephalopods. He found that they appeared first in a ring-like series round the edge of the blastoderm, and subsequently all over the yolk in a layer a little below the surface. He observed their development in the living ovum and found that they "commenced as minute points, gradually increasing in size like other free-formed nuclei." A cell area subsequently forms around them. [53] _Quart. Journ. of Micr. Science_, Vol. XV. pp. 39, 40.