108 NUCLEI OF THE YOLK. 



separated from the yolk. It undergoes segmentation in a perfectly 

 normal fashion. Examples of other cases of this kind have been described 

 by Van Beneden and Bessels 1 in Anchorella, and in Hessia by Van 

 Beneden 2 . It appears from their researches that the protoplasm collects 

 itself together, first of all in the interior of the egg, and then travels to the 

 surface. It arrives at the surface after having already divided into two or 

 more segments, which then rapidly divide in the usual manner to form the 

 blastoderm. 



There are some grounds for thinking that the cases of partial segmen- 

 tation 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 3 . 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. 



Some of the earliest observations on these nuclei were made by Lankes- 

 ter 4 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, gra- 

 dually increasing in size like other free-formed nuclei." A cell area sub- 

 sequently forms around them. 



By E. van Beneden 5 they were observed in a Teleostean ovum to appear 

 nearly simultaneously in considerable numbers in the granular matter 

 beneath the blastoderm. Van Beneden concludes from the simultaneous 

 appearance of these bodies that they develop autogenously. Kupffer at an 

 earlier period arrived at a similar conclusion. My own observations on these 

 nuclei in Elasmobranchii on the whole support the conclusions to be derived 

 from Lankester's, Kupffer's and Van Beneden's observations. As mentioned 

 above, the nuclei in Elasmobranchii do not appear simultaneously, but 



1 Loc. cit. 2 Bulletins de FAcad. Belgique, Tom. xxix., 1870. 



* 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. 

 4 Quart. Journ. of Micr. Science, Vol. xv. pp. 39, 40. 

 6 Quart. Journ. of Micr. Science, Vol. xvm. p. 41. 



