FORMATION OF THE LAYERS. 255 



precisely similar but more partial change in the constitution of 

 the floor takes place in Osseous Fishes 1 . 



The mode in which the floor is formed is a question of some 

 importance. The nuclei, which during the last stage formed 

 a row beneath it, probably, as previously pointed out, take some 

 share in its formation. An additional argument to those already 

 brought forward in favour of this view may be derived from 

 the fact that during this stage such a row of nuclei is no longer 

 present. 



This argument may be stated as follows : 

 Before the floor of cells for the segmentation cavity is formed 

 a number of nuclei are present in a suitable situation to supply 

 the cells for the floor ; as soon as the floor of cells makes its 

 appearance these nuclei are no longer to be seen. From this 

 it may be concluded that their disappearance arises from 

 their having become the nuclei of the cells which form the 

 floor. 



It appears to me most probable that there is a growth in- 

 wards from the whole peripheral wall of the cavity, and that this 

 ingrowth, as well as the cells derived from the yolk, assist in 

 forming the floor of the cavity. In Osseous Fish there appears 

 to be no doubt that the floor is largely formed by an ingrowth of 

 this kind. 



A great increase is observable in the distance between the 

 posterior end of the segmentation cavity and the edge of the 

 blastoderm. This is due to the rapid growth of the latter com- 

 bined with the stationary condition of the former. The growth 

 of the blastoderm at this period is not uniform, but is more rapid 

 in the non-embryonic than in the embryonic parts. 



The main features of the epiblast remain the same as during 

 the last stages. It is still composed of a very distinct layer one 

 cell deep. Over the segmentation cavity, and over the whole 

 embryonic end of the blastoderm, the cells are very thin, 

 columnar, and, roughly speaking, wedge-shaped with the thin 

 ends pointing alternately in different directions. For this reason, 

 the nuclei form two rows ; but both the rows are situated near 

 the upper surface of the layer (vide PI. 7, fig. 5) Towards the 



1 Gotte, "Der Keim d. Forelleneies, " Arch. f. Mikr. Anat. Vol. ix.; Haeckel, 

 "Die Gastrula u. die Eifurchung d. Thiere," Jenaische Zeitschrift, Bd. ix. 



