352 DEVELOPMENT OF ELASMOBRANCH FISHES. 



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 different 

 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 metamor- 

 phosis of adjoining cells, or may not be introduced from else- 

 where. In some of the lower animals, e.g. Hydrozoa, there is no 

 question that the ova are derived from the epiblast; we might 

 therefore expect to find that they had the same origin in Verte- 

 brates. Further than this, ova are frequently capable in a 

 young state of executing amoeboid movements, and accordingly 

 of migrating from one layer to another. In the Elasmobranchs 

 the primitive ova exhibit in a hardened state an irregular form 

 which might appear to indicate that they possess a power of 

 altering their shape, a view which is further supported by some 

 of them being at the present stage situated in a position very 

 different from that which they eventually occupy, and which 

 they can only reach by migration. If it could be shewn that 

 there were no intermediate stages between the primitive ova 

 and the adjoining cells (their migratory powers being admitted) 

 a strong presumption would be offered in favour of their having 

 migrated from elsewhere to their present position. In view of 

 this possibility I have made some special investigations, which 

 have however led to no very satisfactory results. There are to 

 be seen in the stages immediately preceding the present one, 

 numerous cells in a corresponding position to that of the 

 primitive ova, which might very well be intermediate between 

 the primitive ova and ordinary cells, but which offer no suffi- 

 ciently well marked features for a certain determination of their 

 true nature. 



