THE SEGMENTATION OF THE OVUM. IO5 



absent in other cases. It is on the contrary to be regarded 

 merely as a part of the ovum in which the yolk spherules have 

 attained to a very great bulk as compared with the protoplasm ; 

 sometimes even to the complete exclusion of the protoplasm. 



An ordinary meroblastic ovum consists then of a small disc 

 at the formative pole, known as the germinal disc, composed 

 mainly of protoplasm in which comparatively little food-yolk is 

 present This graduates into the remainder of the ovum, being 

 separated from it by a more or less sharp line. This remainder 

 of the ovum, which almost always forms the major part, usually 

 consists of numerous yolk spherules, embedded in a very scanty 

 protoplasmic matrix. 



In some cases, e.g. the eggs of Elasmobranchii 1 , the protoplasm is pre- 

 sent in the form of a delicate network ; in other and perhaps the majority of 

 cases, too little protoplasm is present to be detected, or the protoplasm may 

 even be completely absent. In some Osseous Fishes, e.g. Lota, the yolk 

 forms a homogeneous transparent albuminoid substance containing a large 

 globule at the pole furthest removed from the germinal disc. In this case 

 the germinal disc is sharply separated from the yolk. In other Osseous 

 Fishes the separation between the two parts is not so sharp 2 . In these 

 cases we find adjoining the germinal disc a finely granular material con- 

 taining a large proportion of protoplasm ; this graduates into a material with 

 very little protoplasm and numerous yolk spherules, which is in its turn 

 continuous with an homogeneous albuminoid yolk substance. In Elasmo- 

 branchii we find that immediately beneath the germinal disc there is present 

 a finely granular matter, rich in protoplasm, which is continuous with the 

 normal yolk. 



The Elasmobranch ovum may conveniently serve as type for the Verte- 

 brata. The ovum is formed of a spherical vitellus without any investing 

 membrane. The germinal disc is recognizable on this as a small yellow spot 

 about i^ millimetres in diameter. In the germinal disc a furrow appears 

 bisecting the disc, followed by a second furrow at right angles to the first. 

 Thus after the formation of the second furrow the disc is divided into four 

 equal areas. Fresh furrows continue to rise, and eventually a circular 

 furrow, equivalent to the equatorial furrow of the frog's ovum, makes its 

 appearance, and separates off a number of smaller central segments from 

 peripheral larger segments. In the later stages the smaller segments at first 

 divide more rapidly than the larger, but eventually the latter also divide 

 rapidly, and the germinal disc becomes finally formed of a series of segments 



1 Vide Schultze, Archiv.f. mikr. Anat. Vol. XI.; and F. M. Balfour, Monograph 

 on the Development of Elasmobranch Fishes. 



2 Vide Klein, Quart. Joitrnal of Micr. Science, April, 1876. Bambeke, Mem. 

 Cour. Acad. Belgique, 1875. His, Zeit.fiir Anat. u. Entwicklung. Vol. I. 



