DEVELOPMENT OF ELASMOBRANCH FISHES. 133 



resulted from the division of one primitive ovum, but can hardly 

 be adduced as instances of a commencing coalescence ; since if 

 the ova thus aggregated were to coalesce, an ovum would be 

 produced of a very much greater size than any which is found 

 during the early stages. Though at this stage no indication 

 is present of such a coalescence of cells to form ova as is be- 

 lieved to take place by Gotte, still the origin of the primitive 

 ova is not quite clear. One would naturally expect to find a 

 great number of cells intermediate between primitive ova and 

 ordinary columnar cells. Cells which may be intermediate are 

 no doubt found, but not nearly so frequently as might have 

 been anticipated. One or two cells are shewn in PI. xi. fig. 

 14 a. X, which are perhaps of an intermediate character; but 

 in most sections it is not possible to satisfy oneself that any 

 such intermediate cells are present. 



In one case what appeared to be an intermediate cell was 

 measured, and presented a diameter of *012 Mm. while its 

 nucleus was '008 Mm. Apart from certain features of the 

 nucleus, which at this stage are hardly veiy marked, the 

 easiest method of distinguishing a primitive ovum from an 

 adjacent cell is the presence of a large quantity of protoplasm 

 around the nucleus. The nucleus of one of the smallest primi- 

 tive ova is not larger than the nucleus of an ordinary cell (being 

 about "008 Mm. in both). It is perhaps the similarity in the size 

 of the nuclei which renders it difficult at first to distinguish 

 developing primitive ova from ordinary cells. Except with the 

 very thinnest sections a small extra quantity of protoplasm 

 around a nucleus might easily escape detection, and the de- 

 veloping cell might only become visible when it had attained 

 to the size of a small typical primitive ovum. 



It deserves to be noticed that the nuclei even of some of 

 the largest primitive ova scarcely exceed the surrounding nuclei 

 in size. This appears to me to be an argument of some weight 

 in shewing that the great size of primitive ova is not due to 

 the fact of their having been formed by a coalescence of dif- 

 ferent cells (in which case the nucleus would have increased in 

 the same proportion as the cell) ; but to an increase by a normal 

 method of growth in the protoplasm around the nucleus. 



It appears to me to be a point of great importance certainly 

 to determine whether the primitive ova arise by a meta- 



