Embryology of the River-Lamprey . 33 



especially distinct and lie closer together at the spot at which 

 the furrow has begun to form. 



Before the complete separation of the two segmentation 

 spheres several nuclei appear, and between every two nuclei 

 a spindle becomes visible. It is no doubt partly owing to 

 such a complete separation of the yolk-segments that in the 

 last days of development we find within one egg-membrane 

 two embryos entirely separate from one another, as I fre- 

 quently had occasion to observe. 



The second furrow is likewise longitudinal, and it is only 

 the third that is the first equatorial one. 



After fifteen hours the ovum is represented by a vesicle the 

 upper wall of which consists of a series of smaller cells, the 

 lower of much larger ones. Baer's cavity has attained con- 

 siderable dimensions. 



The ova of tw^enty-eight hours likewise exhibit at the roof 

 of Baer's cavity a single layer of cells. 



At the lower pole, however, there lies a group of larger 

 cells. The transition between the smaller cells of the upper 

 pole and the larger cells of the lower is a gradual one. 



In ova of fifty-five hours Baer's cavity has reached a still 

 larger size. 



The smallest cells lie at the upper pole and already have an 

 epithelial character about them. In many sections the ovum 

 has the appearance of a ring, which at one spot, namely at 

 the upper pole, consists of one layer, while it is bi- or tri- 

 laminate at the other. Sometimes there lies at the lower pole 

 a small group of segmentation-spheres. Even at this period 

 a Rusconi's pit is visible upon the under surface of the ovum. 

 The cells of the outer layer gradually pass into the floor of 

 the pit. The shape of this pit leads us to suppose that it 

 has arisen by the invagination of the outer layer, as is actually 

 the case. This structure is nevertheless a transitory one ; at 

 any rate its appearance very soon changes. The largest cells 

 are found in the pit of Busconi and at its margin. 



When eighty hours have elapsed the outer layer has under- 

 gone still farther modification ; the cells have become still 

 more cylindrical and have almost entirely consumed the 

 yolk-granules. The size of the segmentation-cavity has in 

 the meantime greatly diminished. The whole of the lower 

 half of the ovum consists of small round cells, which, how- 

 ever, are separated by a slight space from the still smaller and 

 much more closely packed cells of the epiblast. The anus of 

 Rusconi can at this time be seen with great distinctness at 

 the lower pole, though it has undergone a change in form and 

 has become narrower and longer. The ovum itself has also 



Ami. & Mag. N. Hist. Ser. 6. Vol. xi. 3 



