SOME POINTS IN EARLY DEVELOPMENT OF HEN S EGG. 165 



that nucleated portions of the bed of white yolk which do 

 not project may also become separated by a process of cleavage 

 and give rise to formative cells. Goette distinguishes these 

 " Dotterzellen " or formative cells from the " Embryonal- 

 zellen " or segmentation cells, and attributes special functions 

 to the former which it is not necessary here to inquire into. 

 He considers that His, Waldeyer, and Oellacher were wrong 

 in concluding that the white yolk is excluded from segmenta- 

 tion, for, he urges, if the floor of the subgerminal cavity be 

 allowed to be white yolk, which he seems to regard as 

 admitting of little doubt, then the white yolk undoubtedly 

 does share in segmentation. However, Goette takes care to 

 state that he does not go so far as to say with His that the 

 formative cells arise from the white yolk in the sense that 

 they are themselves the original elements of the white yolk, 

 but considers them to be new formations occurring in the 

 course of development consequent on incubation. Goette 

 thus commits himself definitely to the statement that formative 

 cells are derived from the white yolk. 



Kolliker (' Entwicklungsgeschichte,' ii, Auflage) does not 

 agree with Goette that the white yolk on the floor of the 

 segmentation cavity shares in segmentation, and objects to 

 Goette's application of the term " white yolk " to the floor 

 of the segmentation cavity which is in contact with the 

 segmentation elements. Kolliker regards this as derived 

 from the germ " Rest des Bildungsdotters," and describes it 

 as finely granular and containing no true elements of the 

 yolk. He also states that large elements arise from this floor 

 as outgrowths from the general surface, but does not attach 

 much importance to it, nor does he give further grounds for 

 regarding this layer as germinal in nature. 



Great light has been thrown on this subject by researches on 

 the ova of fishes — by Van Bambeke and Klein on Teleostean 

 fishes, and by Balfour on Elasmobranchs. Van Bambeke 

 describes the subgerminal Ijfyer, or " couche intermediare," 

 as raised up at either end of the germ in the form of a thick 

 pad, " bourrelet," which sends in from both sides a thin layer 

 towards the centre. The thickness of the central part varied 

 with the period of development, becoming thicker as develop- 

 ment proceeded. From the study of different stages, van 

 Bambeke concludes that this peripheral thickening of the 

 *' couche intermediare " is derived from the germ in the first 

 instance and grows inwards towards the centre, where the 

 processes of the two sides meet and coalesce. Van Bambeke 

 describes this layer as richly nucleated, especially its peri- 

 pheral thickenings. The material forming the substance 



