86 



Primitively, all ova have such an interlacing protoplasmic net- 

 work culminating in a superficial sheet. In the case of holoblastic 

 eggs this structure is retained. In the egg of the lamprey, for 

 example, the interlacing or vacuolated cytoplasm may be easily 

 traced from the clear space around the nucleus into the region 

 beyond, which is thickly beset with oval yolk granules, and also 

 into the peripheral layer. This is also evident from the figures and 

 description of Herfort.* How far the primitive protoplasm of the 

 3^olk in meroblastic eggs is retained is not yet quite clear, but in the 

 neighbourhood of the nucleus it is certainly present, and can be 

 followed into the 3'olk, and it also forms a superficial layer. A 

 preparation I possess of a fully developed egg of Acanthias from the 

 ovary shows both these features. In meroblastic eggs a so-called 

 periblastic extension of the protoplasm outwith the segmentation 

 area is a recognition of the existence of the peripheral Isijer of proto- 

 plasm. The presence of such a layer and of fine strands of proto- 

 plasm amongst the j^olk granules of such large meroblastic eggs as 

 those of reptiles and birds has been described by Waldeyer,f SarasinJ 

 and His. 1 1 It has still to be determined whether the streaming of 

 the protoplasm towards the animal pole of such large meroblastic 

 eggs removes the protoplasm entirely from the vegetal region of the 

 egg or reduces the strands to such a degree of tenuity as to make 

 them difficult to follow. 



Segmentation. — The description of the segmentation of the 

 egg of the Teleosteans given by Agassiz and Whitman § is exactly 

 that which I have observed in the case of pelagic Teleostei. The 

 procedure, as these authors stated, is subject to modification. There 

 is one feature which these authors certainly saw and which led later 

 to some extent to Whitman's joaper on the " Inadequacy of the Cell 

 Theory," ** but which does not appear to have received the attention 

 it deserves — the remarkable and yet evidently general method of 

 cleavage, ft The cells are not completely separated from one another. 

 After the nuclei have separated vacuoles appear in the region pre- 

 viously occupied by the equator of the spindle, and these are added 

 to so as to bring about a line of demarcation between the cells, but 

 which, nevertheless, remain connected by at first thick laminae and 

 latterly by thin protoplasmic bridges. 



* 1893, Anat. Anz., Bd. 8. 1900, Arch. f. Aiiat. u. Entw.-ges., Bd. 57. 



t 1870, Eierstock u. Ei. 1906, Hertwig'a Haiidb. d. Entw. d. Wiibeltiere, Bd. 1. 



t 1883, Inaug. Diss. 1900, Histog. Studien. § Loc. cit. 



** 1893, Wood's Hole. Lectures. Also 1894, Joiir. of Morph., v., 9. 



ft See also Brook, Proc. Eoy. Phys, Soc, Edin., 1886. 



