OP ARTS AND SCIENCES. 27 



entiation are questions which have not been exhausted. Some of the 

 general features of the cleavage have been understood since the time 

 of Rusconi ; ■'^ and the researches of Ryder and Hoffmann have 

 demonstrated the existence of polar globules, pronuclei, and karyo- 

 kinetic figures, in addition to numerous other facts of both special 

 and general importance. But it is manifest that our knowledge in 

 this direction, invaluable as it is, is very far from having reached that 

 degree of completeness with which we are familiar in the case of some 

 other vertebrates, and many invertebrates. The importance of accu- 

 rate and detailed study of the cleavage phenomena has been illus- 

 trated in so many cases in recent years, that it is now fast becoming 

 unnecessary to insist upon it. 



Remembering that the histogenetic sundering of the embryonic 

 material actually begins with the cleavage, and that "jeder einzelne 

 Entwicklungsmomeut ist die nothwendige Folge des vorausgegange- 

 nen und die Bedingung des folgenden," ''^ it seems clear what course 

 our investigations should take in order to reach satisfactory conclu- 

 sions on the origin and relation of the germ-layers. But in all 

 telolecithal vertebrate ova, especially those extreme forms in which 

 the cleavage is restricted to a discoidal mass aggregated at one pole, 

 the difficulties in the way of tracing the precise genealogy of indi- 

 vidual cells soon become quite insurmountable. Notwithstanding the 

 exceptional advantages for observation afforded by transparent pelagic 

 fish eggs, no one has hitherto succeeded in tracing the exact genetic 

 relationship of each cell beyond the 1 6-cell stage. 



In passing from the 1 6-cell to the 32-cell stage, the central portion 

 of the blastodisc becomes two cells deep, and on this account it be- 

 comes extremely dilficult, beyond the latter stage, to trace the genesis 

 of the individual cells in the living egg. By the aid of mounted prep- 

 arations we have found it possible to obtain the complete genealogical 

 liistory of each cell as far as the G4-cell stage. In leaving this stage 

 the blastodisc becomes three cells deep in its central portion, and we 

 have been unable to carry the complete identification of all the cells 

 beyond this point. Fortunately, the more interesting among the con- 

 cluding phenomena of the cleavage are confined to the marginal cells 

 of the disc ; and it is the history of these cells that we have been 

 able to follow with sufficient completeness to decide one of the 

 cardinal questions in the early development of the teleostean fishes, 



11 Mailer's Arch., 1836, p. 278. 



1- Leuckart and Berginann. Vergl. Anat. u. Phys. d. Thierreiches, p. 19. 



