II 



SEGMENTATION AND GASTRULATION 



121 



Ctenophora. Holoblastic centrolecithal eggs : Geryonia (Craspcdote, Medusa}. Mero- 

 blastic telolecithal eggs : Shark. Meroblastic centrolecithal eggs : Insects. 



The first divisions occur in directions which seem to be identical in the eggs of 

 most animals. The first plane of division which, after previous nuclear division, 

 separates the two daughter cells of the egg, i.e. the first two segmentation spheres, 

 or blastomeres, runs in the direction of the principal axis of the egg, from the 

 animal to the vegetative pole. By the animal pole we mean that portion of the 

 surface of the egg at which the spermatozoon entered, and near which, in telolecithal 

 eggs, the chief mass of the formative yolk lies. The point lying diametrically opposite 

 to this we call the vegetative pole. The first plane of division is, therefore, witli 

 relation to the two poles, meridional. 



The holoblastic alecithal egg falls at the first division into two similar blasto- 

 meres, each of which has a nucleus in its centre. 



The holoblastic telolecithal egg falls generally into two blastomeres, each of which 

 repeats the structure of the undivided egg. Each blastoniere shows polar differentia- 

 tionat the animal pole lies the greatest mass of formative yolk with the nucleus, 

 at the vegetative pole the greatest mass of nutritive yolk (Fig. 89, B, C). 



The holoblastic centrolecithal egg (Geryonia) falls into two blastomeres, each of 

 which repeats the structure of a telolecithal egg, the formative yolk only appearing 



FIG. 89. 2-Blastomere stage of different eggs. These are placed in this arid all following 

 illustrations, so that the animal pole is directed upwards, and the vegetative pole downwards. The 

 nucleus is black, the formative yolk dark, the nutritive yolk light and granulated. F represents 

 merely a portion from the animal pole of an egg. 



at the free surface of the blastoniere, and not at the side which is directed towards 

 the plane of division. The nucleus of each blastomere lies superficially in the forma- 

 tive yolk (Fig. 89, D). 



In the meroblastic telolecithal egg the inert lifeless nutritive yolk is so strongly 

 developed in comparison with the living active formative yolk, that the formative 

 yolk when dividing is not able to effect the division of the whole of the nutritive 

 yolk. The former only therefore divides, the latter remaining undivided. We have 

 thus a large sphere of nutritive yolk, with two masses of formative yolk divided by a 

 meridional furrow at the animal pole, in each of which lies a nucleus (Fig. 89, F). 



In the meroblastic centrolecithal, or rather mesolecithal, egg the central forma- 

 tive yolk again alone is able to divide under the influence of the nucleus, while the 

 remaining portions of the egg continue at first undivided (Fig. 89, E). 



The second plane of division is also meridional and stands at right angles to the 

 first. It divides each of the first blastomeres into two halves exactly in the same 

 way as the first plane divided the whole egg. 



The third plane of division seems to be pretty generally equatorial. It is visible 

 on the exterior of the egg as an equatorial furrow. It stands at right angles to the 

 first two planes of division and to the chief axis of the egg, and divides the first 4 



