THE SEGMENTATION OF THE OVUM. 109 



increase in number as development proceeds ; and it is possible that Van 

 Beneden may be mistaken on this point. No evidence came before me 01 

 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 1 , 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 obser- 

 vations do not however appear to be very conclusive. 



It must be admitted that the general evidence at our com- 

 mand appears to indicate that the nuclei of the yolk in mero- 

 blastic 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 segmen- 

 tation nucleus. 



The existence of these nuclei in the yolk and the formation 

 of a distinct cell body around them is a strong piece of evidence 

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

 accepted,) that the part of the ovum of meroblastic ova which 

 does not segment is of the same nature as that which does 

 segment, and differs only in being relatively deficient in active 

 protoplasm. 



The following forms have meroblastic ova of the first type : the Cephalo- 

 poda, Pyrosoma, Elasmobranchii, Teleostei, Reptilia, Aves, Ornithodelphia (?). 

 The second type of meroblastic segmentation occurs in many Crustacea, 

 (parasitic Copepoda, Isopoda Mysis, etc.). It is also stated to be found in 

 Scorpio. 



The ova of the majority of groups in the animal kingdom 

 segment according to one of the types which have just been 

 described. These types are not sharply separated, but form an 

 unbroken series, commencing with the ovum which segments 

 uniformly, and ending with the meroblastic ovum. 



1 At the time when my observations on Elasmobranchii were carried out, this 

 peculiar condition of the nucleus had not been elucidated. 



