80 PARTIAL SEGMENTATION. 



part of meroblastic ova which does not undergo segmentation is not 

 a new addition 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. y. the eggs of Elarsmobranchii ', the protoplasm is 

 present iii 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. y. 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 containing a large proportion of protoplasm; this graduates into 

 a material with very little protoplasm and numerous yolk spherules, A^ich 

 is in its turn continuous with an homogeneous albuminoid yolk substance. 

 In Elasmobraiichii 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 

 Vertebrata. The ovum is formed of a spherical vitellus without any in- 

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

 yellow spot about 1-J,- 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 oif 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 of a fairly uniform size. So much 

 may be observed in surface views of the segmenting ovum, and it may be 

 noted that there is not much difference to be observed between the se<>'- 



o 



mentation of the germinal disc of the Fowl's ovum and that of the Elasmo- 

 brauchii. Indeed the figure of the former (fig. 44) would serve fairly well 

 for the latter. When however we examine the segmenting germinal discs by 



1 Vide Sckultze, Arclilv f. mikr. Anut. Vol. xi. ; and F. M. Balfour, Monograph on the 

 Development of Elasmobranch Fishes. 



Vide Klein, Quart. Journal of Micr. Science, April, 1876. Bambeke, Mem. Coitr. 

 A cud. Rel<ji<iue, 1H75. Hit, Zeit.fiii' Anat, u. Entwicklung . Vol. i. 



