SEGMENTATION OF THE EGG. 19 



occur, and by the fact that the cells, or blastomeres, into which the 

 egg is divided are approximately equal to one another in size. 

 . The first cleft, Fig. 2, n, is a vertical one, and divides the egg 

 into two perfectly similar halves. The second cleft is also 

 vertical, but at right angles to the first one : on its completion 

 the egg is divided into four cells or blastomeres of equal size, 

 Fig. 2, in. The third cleft, Fig. 2, IV, is a horizontal one, and 

 divides each of the four blastomeres of the previous stage into two, 

 of which the lower one is slightly the larger. Two vertical 

 clefts next appear simultaneously, at angles of 45 with the two 

 first clefts : by these the number of the blastomeres is again 

 doubled, giving sixteen in all, Fig. 2, v. Two new horizontal 

 clefts double the number of blastomeres once more ; the stage, 

 with thirty-two blastomeres, being shown in Fig. 2, vi. From 

 this time segmentation continues rapidly, but with less regu- 

 larity : later stages are shown in Fig. 2, vn and vm. 



Segmentation is said to be complete, or holoblastic, when, as 

 in Amphioxus, the whole egg is divided up at once into blasto- 

 meres : it is further distinguished as equal when, as again in 

 Amphioxus, the several blastomeres are from the first approxi- 

 mately equal in size. 



In the frog's egg, Fig. 3, segmentation is holoblastic, but 

 unequal. The first two clefts, which, as in Amphioxus, are vertical, 

 divide the egg equally and symmetrically ; but the third, or hori- 

 zontal cleft, Fig. 3, in, is much nearer the upper than the lower 

 pole, and throughout the later stages of segmentation, Fig. 3, 

 IV and v, there is marked inequality in size between the blasto- 

 meres of the upper and lower halves of the egg. Unequal seg- 

 mentation is due to food yolk, which, in a telolecithal egg like 

 the frog's, is specially accumulated in the lower pole, and retards 

 the developmental processes in this as compared with the upper 

 half of the egg. 



An exaggeration of this condition is seen in the hen's egg, 

 in which food-yolk is present in such quantity as to absolutely 

 stop the processes of development in all parts of the egg, except 

 in a small circular patch on the surface, corresponding to the 

 upper pole of the egg of Amphioxus or the frog. To this 

 circular patch, or germinal disc, Fig. 4, BA, segmentation 

 is restricted. Figs. 5 and 6 represent surface views of the 

 germinal disc during the process of segmentation, and show the 



c 2 



