HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 13 



five minutes apart, beginning with the fresh-laid unfertilized egg, which is 

 followed by twelve views of a fertilized egg. The stage reached at the 

 end of 60 minutes (Fig. 52) represents the fully formed blastodisc, as it 

 appears just before the cleavage begins. Figs. 53-63 represent another 

 series of views of an egg taken from the sea, beginning about 30 minutes 

 after fertilization and ending with the first cleavage (75 min.). 



The unfertilized egg (Fig. 40). a few moments after contact with the 

 water, has already cleared up sufficiently for an optical section of the 

 cortical layer. No trace of a disc is to be seen, but the layer is thicker 

 at the micropylar pole than elsewhere, being here 15 jjl thick, and thin- 

 ning out gradually towards the opposite pole, where it is not more than 

 3—") jx. The granules are spherical, and vary from 1 to 6 p. in diameter. 

 The micropyle is funnel-shaped, 15 /j. in length, with a mouth 4 /x in diam- 

 eter. The inner end of the micropyle lies in the cortical layer, and it* 

 orifice cannot be seen. A thin space is now seen between the egg and 

 its membrane. 



The condition five minutes after fertilization (Fig. 41) is but little 

 changed. The polar thickening of the cortical layer has slightly increased, 

 but no disc is yet seen. A considerable polar area is now in close contact 

 with the membrane around the micropyle, while elsewhere a perivitelline 

 space is seen. It is difficult to determine whether the micropyle has an 

 eccentric position at this time, but there is no doubt on this point in the 

 next stage (Fig. 42). A plain discoidal thickening is now apparent, the two 

 laces of which are about equally convex. The disc has retreated from 

 the micropyle, and on its outer face, lying a little eccentric, is seen the 

 first polar globule. Five minutes later (Fig. 43), the blastodisc has doubled 

 its thickness, the perivitelline space is much enlarged, and the polar 

 globule comes boldly into view, resting on a low prominence that appears 

 as the half-formed second polar globule in the next stage (Fig. 44). The 

 blastodisc is now considerably thicker, while the periblast, at the expense 

 of which the blastodisc is forming, has become sensibly thinner. In the 

 next phase (Fig. 45) the perivitelline space about the formative pole ba- 

 its greatest width, and the inner convexity of the blastodisc. which has 

 been steadily growing, has attained its maximum prominence. This stage 

 is usually reached in about 3(1 minutes, and it corresponds to that seen in 

 Fig. 53, where the inner convexity is still more pronounced. The outer 

 face of the disc in this c^^ (Fig. 45) has a convexity that does not per- 



