42 FORMATION OF THE LAYERS. 



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 PL III. 

 fig. 5). Towards the posterior end of the blastoderm the cells 

 are flatter and broader ; and the layer terminates at the non- 

 embryonic end of the blastoderm without exhibiting the slight- 

 est tendency to become continuous with the lower layer cells. 

 At the embryonic end of the blastoderm the relations of the 



