DEVELOPMENT OF HALISARCA LOBULARIS. 607 



occasionally visible^ now stand out as a striking deeply -stained 

 spot (nucleolus) surrounded by a clear circular space (nucleus). 



Segmentation having now resulted in the formation of a gene- 

 rally oval morula, consisting of a multitude of small cells em- 

 bedded in a structureless blastema, a curious change follows. 

 The several cells begin to collect together in irregular strings and 

 heaps, forming a rude kind of network, the meshes of which 

 are filled with the structureless blastema from which they have 

 separated out. The process of cell aggregation continues till 

 the cells have for the most part arranged themselves in a single 

 external layer, forming the foundation of a blastula. At the 

 same time a large number of the cells remain accumulated in 

 heaps at various points on the interior of the blastula-wall. 

 Before these have disappeared an infolding of the wall occurs 

 at one or more points at those places, namely, where the 

 residual cells of the interior are chiefly aggregated. 



As the process of folding continues the interior of the 

 blastula-liKe body soon becomes cleared of the cell-stragglers, 

 in what precise manner, whether by their dissolution or by 

 their deploying into line with the rest of the cells in the wall, 

 I have no means of determining. 



The singular mode of formation of the blastula and gastrula 

 no doubt stands in close connection with the absence of a seg- 

 mentation cavity in the embryo, and may be most probably 

 accounted for as due to " abbreviation '' in adaptation to 

 development in a confined space. The blastula is not formed 

 by the enlargement of a segmentation cavity, since this other- 

 wise empty space can be more advautageously occupied by cells 

 which subsequently become utilised in the formation of the 

 gastrula. Directly a cavity becomes necessary, in order to 

 render possible the infolding which leads to the gastrula stage, 

 the loosely-aggregated cells of the morula pack themselves 

 closely together to form the wall of the unfinished blastula; but 

 as this does not aff'ord sufficient room for them all, a part 

 remain loosely aggregated in the interior, and these subse- 

 quently arrange themselves into a unicellular layer along the 

 line of the infolding wall of the gastrula of which they then 



