HISTOltY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 15 



the other, by additions at any or all points of its inferior (internal) sur- 

 face. The two modes may be distinguished as marginal and subgernunal. 

 There is no radical difference between tlu'in, as the marginal growth is 

 evidently derived from and included in the subgertninal. Marginal growth. 

 is perhaps predominant in all eases of subgerminal growth, and it has 

 certainly acquired an exclusive role in the majority of pelagic fish eggs. 

 This fact borne in mind will help to reconcile many conflicting state- 

 ments regarding the origin and history of the periblast. 



We are entirely in the dark as to the real nature of the movements 

 which give rise to the blastodisc, and have to content ourselves with the 

 simple description of the phenomena. We know that in those cases where 

 the yolk elements have melted together in the form of a homogeneous ball, 

 as in the egg of Ctenolabrus, the formative material is usually confined 

 to the periphery of the egg sphere; and that where the yolk spherules 

 do not so melt together, but remain distinct, the formative material is 

 in part peripheral, in part diffused among the yolk spherules. When 

 the time arrives for the formation of the disc, the movement of the 

 formative material towards the pole becomes general. The movement 

 may express itself in a steady, even flow that is quite imperceptible, and 

 becomes evident only through the resulting disc and the thinning of 

 the periblast, or in visible streams converging to the disc. Such streams 

 have been observed in the egg of the cod by Ryder (No. 4, p. 17), and 

 described as follows: '-During its migration the germinal protoplasm 

 eventually arranges itself in radiating bands, which sometimes anastomose, 

 and all trend towards and join the edge of the incipient disc. Later, these 

 bands develop nodes or enlargements along their courses, as shown in 

 Fig. 7, and pour their substance into the disc. ... If a granule in one 

 of these bands is watched for a time, it will be noticed that it exhibits 

 a more or less decided progressive movement." Ryder also describes 

 "waves of contraction" that accompany the formation of the disc. 



Movements of a similar nature have been described by many authors, 

 and an excellent review of the observations made in this direction may 

 In' found in Van Bambeke's researches." One of the most interesting and 

 detailed accounts is given by Hansom. 8 



7 f'li. Van Bambeke. llecherches sur PEmbryologie des Toissons Ossoux. Mem. de l'Acad. Belg., 

 \L. 1376. 



■ \\ . II Ransom. Observations on the Ovum of Osseous Fishes. Phil. Tnuis. Roy. Soc. Loud., 

 CLVII. p. 131, ISB7- 



