xvr. 



THE BLASTOPOUE OF THE FROG'S EGG IN RELATION TO 



THE HYPOBLAST. 



By J. B. EussELL, B.Sc. 



£.ead at Watford, 23rd April, 1895. 



PLATE VIII. 



Before I refer to the special subject which I desire to bring 

 before the Society with regard to the egg of the common frog 

 {Rana temporaria), it is necessary to consider some of the earlier 

 stages in its development. 



The process of cell-division, or segmentation as it is called, 

 of the frog's egg, is very similar to that which takes place in 

 Amphioxus, but there are important differences which are due 

 in great measure to the amount and distribution of the food-yolk. 



This food-yolk, which is much more abundant in the lower 

 hemisphere than in the upper, consists of nutritious matter em- 

 bedded in the substance of the egg, and although this forms a 

 ready store of nutriment for the developing embryo, yet it greatly 

 impedes and interferes with the symmetrical segmentation of the 

 egg, as will be seen from Figures 2, 3, and 4, in Plate VIII. 



The egg, just before the completion of the first cleft, dividing it 

 into two equal parts, is represented in Fig. 1 . It will be observed 

 that the cleft is at this stage incomplete below, in consequence 

 of the presence of a large amount of food-yolk. A small cavity, 

 also, has made its appearance in the interior : this is the segmenta- 

 tion-cavity (S). After this a second cleft is formed at right angles 

 to the first ; and Fig. 2 shows the third cleft, which is equatorial 

 and much nearer the upper pole than the lower. 



After this stage the division of the egg is continued according 

 to no regular plan, but it will be seen from the figures that the 

 upper cells are smaller than those at the lower pole. This is of 

 course due to the absence of the hampering effect of yolk-cells. 

 The ovum at the close of segmentation is represented in Fig. 4. 

 At this stage we have a ball of cells — the upper ones, pigmented 

 and devoid of yolk, fonning the primary epiblastic layer ; the lower 

 ones large yolk-bearing cells, and unpigmented. These surround 

 the segmentation-cavity. 



In the succeeding stages of development the cpiblast (Ep, Fig. 5) 

 gradually encircles the egg until only a small circular patch remains 

 at the lower pole (LB-LB', Fig. 6). The growth of the epiblast 

 in this way takes place by the division of yolk-cells into smaller 

 pigmented ones at the surface. 



The point to which I wish to direct special attention is that the 

 alimentary cavity is formed as a narrow slit-like aperture opening 

 at LB, Fig. 5. I have placed a section under the microscope 

 showing this point. The slit rapidly grows inwards and spreads 

 out beneath the surface of the egg near the future dorsal surface 



VOL. VIII. PART VI. 10 



