SEGMENTATION OF THE EGG. 51 



A pause of about an hour now ensues, and then the second 

 cleft is formed. This also is vertical, but in a plane at right 

 angles to the first ; it bisects each of the two first blastomeres, 

 and so gives rise to four equal and similar blastomeres (Fig. 14, 

 in) ; these are ovoid in shape, with their apposed surfaces slightly 

 flattened by mutual pressure. 



The third cleft, which appears about a quarter of an hour 

 later, is a horizontal one, dividing each of the four blastomeres 

 of the previous stage into two (Fig. 14, iv). The cleft is a little 

 above the equator, so that the four blastomeres of the upper tier 

 are a little smaller than those of the lower tier. The blastomeres 

 are in contact with one another laterally, but do not quite meet 

 along the axis of the embryo. Hence the embryo is at this 

 stage in the form of a ring, or short tube, with a central cavity, 

 the segmentation cavity or blastocoel, which at present is open 

 at both the upper and lower poles. 



About a quarter of an hour later, the number of the blasto- 

 meres is again doubled by two new vertical clefts, which appear 

 simultaneously, in planes at right angles to each other, and at 

 angles of 45 with the first two clefts. The embryo now consists 

 (Fig. 14, v) of sixteen blastomeres, arranged in an upper tier of 

 eight rather smaller ones, and a lower tier of eight rather 

 larger ones. 



A little later, about three hours from the time of fertilisa- 

 tion, two more horizontal clefts appear simultaneously, dividing 

 each of the tiers into two, and again doubling the number of the 

 blastomeres. The embryo now (Fig. 14, vi) consists of four tiers, 

 each of eight cells ; the cells of the lowest tier, as. shown in the 

 figure, being decidedly larger than those of the other tiers. The 

 blastocoel (Fig. 14, vi, B) still opens to the exterior at both poles, 

 although the apertures are considerably narrowed by approxi- 

 mation of the cells of the upper and lower tiers respectively. 



In the next stage (Fig. 14, vn)the lowest tier of blastomeres 

 of the preceding stage has divided horizontally, giving five tiers 

 in all ; and each of the blastomeres of the four upper tiers has 

 divided vertically into two. The embryo now, as shown in the 

 figure, consists of five tiers of blastomeres, the four upper of 

 which consist each of sixteen blastomeres, while the lowest tier 

 consists of eight much larger blastomeres. The larva is nearly 

 spherical in shape, and by approximation of the blastomeres of 



E 2 



