18 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



of the syncytium are seen to be of a different character. They are 

 now of enormous size and of peculiar lobed appearance. The lobing 

 becomes more complex as time goes on and appears to be due to 

 incomplete and irregular attempts at amitotic division. 



The discussions, alluded to above, have centred round the mode 

 of origin of these highly characteristic giant nuclei. Balfour, who 

 first described them (1874), did not express any opinion as .to their 

 origin. Riickert in his first paper (1885) on Elasmobranch develop- 

 ment looked on them simply as specialized embryonic nuclei and 

 gave the masses of protoplasm in which they are embedded the name 

 "merocytes." Latterly however Riickert, after his discovery of 

 polyspermy in Eiasmobranchs, has taken the view that the yolk- 

 nuclei are really the accessory sperm-nuclei before alluded to which 

 have altered their character in correlation with the altered environ- 

 ment in which they find themselves after leaving the germinal disc. 



In spite of Riickert's more recent observations and conclusions, 

 and in spite of their being supported by Samassa, Beard and others, 



Fig. 10. — Views of the segmenting germinal disc of Bdellostoma stouti. 

 (After Bashford Dean, 1899.) 



it must, I think, be admitted that the sperm-origin of the yolk-nuclei, 

 is by no means demonstrated. And all general considerations are in 

 favour of Riickert's earlier view being the correct one, namely that 

 the nuclei of the yolk-syncytium are genetically of the same order 

 as the ordinary embryonic nuclei. Such general considerations 

 render it extremely improbable that accessory gamete nuclei should 

 really play an important physiological part in the developing embryo : 

 it is far more probable that such nuclei simply persist for a time, 

 undergo mitosis a few times and then degenerate and disappear. 



The variations in the process of segmentation are well illustrated 

 by the three cases just described and it will be convenient now 

 to summarize the general characteristics of the process in the various 

 remaining groups. 



Lampreys. — In , the Lamprey the phenomena of segmentation 

 agree closely with those observed in the frog and need not be further 

 described. 



Myxinoids. — In the Myxinoids the somewhat sausage-shaped 

 egg is heavily yolked and possesses a germinal disc situated close 

 to one pole. A few segmentation stages of Bdellostoma (Fig. 10) 



