88 NUCLEI OF THE YOLK. 



the yolk 1 . At impregnation this withdraws itself from the yolk. It is to 

 be compared to the protoplasmic network of the Elasmobranch ovum. 



A 





^v-^^SZ^s&Ps 



EliT'a'gTgTMrja ~ l str^e' 



FlG. 47. Two LONGITUDINAL SECTIONS OP THE BLASTODERM OF A PpvISTIUKUS EMBRYO 

 AT STAGES PRIOR TO THE FORMATION OF THE MEDULLARY GROOVE. 



cp. epiblast; //. lower layer cells; m. mesoblast; hi/, hypoblast; so. segmentation 

 cavity; e s. embryo swelling; n'. nuclei of yolk; e. r. embryonic rim. 



There are two types of meroblastic ova. In one of these (Aves, 

 Elasmobranchii) the germinal disc is formed in the ovarian ovum. 

 In the second type the germinal disc is formed after impregnation 

 by a concentration of the protoplasm at one pole. This concentration 

 is analogous to what has already been described for Anodon and 

 other Molluscan ova (p. 82). 



The ova of some Teleostei are intermediate between the two types. 



The ovum of the wood-louse, Oniscus murarius 2 , may he taken as an 

 example of the second type of meroblastic ovum. In this egg development 

 commences by the appearance of a small clear mass with numerous 

 transparent vesicles. This mass is the protoplasm which has become 

 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 3 in Anchorella, and in Hessia by Van Beneden 4 . 

 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 A rthropoda 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 blasto- 

 derm, around which cells become differentiated. (Fig. 46 and 47.) 

 These cells join the part of the blastoderm formed by the normal 



1 Vide. Bambeke, loc. cit. 



' Vide Bobret/kv, Zeitxchrift fiir -iriss. Zoolor/ie, Vol. xxiv., 1874. 



3 Loc. cit. 4 Bulletins </< VAcad. Rclfiiqiie, Torn, xxix., 1870. 



