L OVOGENESE DES SELACIENS ET DE QUELQUES AUTRES CHORDATES 25 



unicellulaire et n'exigerait pas pour se différencier le sacrifice des autres 

 cellules du même nid. Cette manière de voir nous paraît être celle de Wal- 

 deyer (1870-1903); elle est partagée, en ce qui regarde les batraciens, par 

 Semon (1892) et Eismond (1898), qui pourtant admettent conjointement les 

 deux autres modes. 



Quelle est l'évolution des nids cellulaires che\ les sélaciens? 



Le processus décrit par Balfour (1878) a pris place tel quel dans les 

 traités classiques, par exemple dans le Traité de Zoologie d'EDMOND Per- 

 rier ffasc. VI, 1903) et ailleurs. Comme il est souvent incomplètement rap- 

 porté par les auteurs, nous croyons bien faire en empruntant à Balfour 

 lui-même l'exposé qu'il en donne dans son Treatise on comparative Em- 

 bryology (vol. I, 1880). » It is convenient, - écrit-il, « to distinguish tn>o 

 modes in which the primitive germinal cells may become converted into per- 

 manent ova, though the morphological différence between the two modes 

 is of no great importance. 



- In the first mode, the protoplasm of ail the cells forming a nest 

 unités into a single mass containing the nuclei of the previously indepen- 

 dent ova. The nuclei in the nest increase in number, probably by division, 

 and at the same time the nest itself increase in size. The nuclei while 

 increasing in number also undergo important changes. A ségrégation of 

 their contents takes place, and the granular part (nuclear substance) forms a 

 mass close to one side of the membran of the nucleus, while the remainder 

 of the nucleus is filled with a clear fluid. The whole nucleus at the same 

 time increase somewhat in size. The granular mass gradually assumes a 

 stellate form, and finally becomes a beautiful reticulurn of the character so 



well known in nuclei Not ail the nuclei undergo the above changes; 



but some of them stop short in their development, undergo atrophy, and 

 appear finally to be absorbed as pabulum by the protoplasm ofthe nest .... 



Thus only a few nuclei out of a nest undergo a complète development 



The relative number of ova ivhich may develope from a single nest is sub- 

 ject to great variation. The object ofthe whole occurrence of the fusion of 

 primitive ova and the subséquent atrophy of some of them is to ensure the 

 adéquate nutrition of a certain number of them. 



» In the second and rarer mode of development of permanent ova from 

 primitive germinal cells, the nuclei and protoplasm undergo the same 

 changes as in the first mode, but the cells either remain isolated, and never 

 form part of a nest, or form part of a nest in which no fusion of protoplasm 



