THE DEVELOPMENT OF LEPIDOSIREN PAHADOXA. 17 



during the process of gastrulation, does not by any means 

 keep time witli the latter — a further support to my asser- 

 tion that the former is not the direct cause of the latter. 



What takes place may be said to be in general terms that 

 the floor of the segmentation cavity is brought up against 

 its roof. During this process, however, a transient phase 

 occurs which is not without interest. While the cavity is 

 still at its full development we notice a tendency for large- 

 yolk blastomeres to become arranged round the segmentation 

 cavity, and in close contact with its roof (cf. PI. 2, fig. 8 ; 

 or better, figure of Protopterus VIII); following this, 

 the smaller blastomeres lying in and near the floor of the 

 cavity push out processes, become irregular and angular in 

 shape, and, attaching themselves to one another by their 

 corners, form a loose and irregular sponge-work traversing 

 the cavity completely (PI. 3, fig, 11). As will be gathered 

 from the figures, the segmentation cavity during this pro- 

 cess, although broken up by the sponge-work, really extends 

 through a much larger volume than it did before. As, hoAV- 

 ever, gastrulation proceeds, the fluid filling the meshes of 

 the sponge-work becomes absorbed, and the blastomeres 

 resume their spherical or, as they become pressed closer 

 together, polyhedral shape. We may still for a long time, 

 however, observe chinks persisting here and there, especially 

 laterally. The roof cells of the segmentation cavity remain 

 all through the stages we are now describing sharply marked 

 off from the large yolked elements which have been laid up 

 against them. 



Origin of the Mesoblast and Notochord. — PI. 3, 

 fig. 14, illustrates a section through an egg of Stage 12 and 

 transverse to the axis of the medullary plate region. Lying- 

 over the archenteron and tapering off on each side is a mass 

 of cells distinguished from the remainder of the inner cells 

 by their smaller size, more finely granular yolk, and by their 

 rounded form. Immediately over the archenteron these 

 small cells are aggregated closely together, but laterally as 

 a rule they are separated by wide chinks — the remains of the 



VOL. 45, PART 1. NEW SERIES. B 



