THE SEGMENTATION OF THE OVUM. 89 



segmentation of the germinal disc. Such nuclei are formed in all 

 craniate meroblastic ova 1 . In Cephalopoda 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 2 in the Cephalopoda. 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 develop- 

 ment 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. 



By E. van Beneden 3 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 simul- 

 taneous appearance of these bodies that they develope autogenously. 

 Kupffer at an earlier period arrived at a similar conclusion. My own 

 observations on these nuclei in Elasniobranchii on the whole support the 

 conclusions to be derived from Lankester's, Kupffer's and Van Benedeu's 

 observations. As mentioned above, the nuclei in Elasmobranchii do not 

 appear simultaneously, but increase in number as development proceeds ; 

 and it is possible that Van Beneden may be mistaken on this point. No 

 evidence came before me of derivation from pre-existing nuclei in the 

 blastoderm. My observations prove however that the nuclei increase by 

 division. This is shewn by the fact that I have found them with the 

 spindle modification (fig. 46, nx"), and that in most cases they usually exhibit 

 the form of a number of aggregated vesicles 4 , which is a character of 

 nuclei which have just undergone division. It should be mentioned 

 however that I failed to find a spindle modification of the nuclei in the 

 later stages. Against these observations must be set those of Bobretzky, 

 according to which the nuclei in Oniscus are really the nuclei of cells which 

 have migrated from the blastoderm. Bobretzky 's observations do not 

 however appear to be very conclusive. 



It must be admitted that the general evidence at our command 

 appears to indicate that the nuclei of the yolk in meroblastic ova 

 originate spontaneously. There is however a difficulty in accepting 

 this conclusion in the fact that all the other nuclei of the embryo are 

 descendants of the first segmentation nucleus; and for this reason 

 it still appears to me possible that the nuclei of the yolk will be 

 found to originate from the continued division of one primitive 

 nucleus, itself derived from the segmentation nucleus. 



The existence of these nuclei in the yolk and the formation of 

 a distinct ceil body around them is a strong piece of evidence in 

 favour of the view above maintained, (which is not universally 



1 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. 

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



3 Quart. Journ. of Micr. Science. Vol. xvm. p. 41. 



4 At the time when my observations on Elasmobrauchii were carried out, this peculiar 

 condition of the nucleus had not been elucidated. 



