SUBKINGDOM METAZOA. 53 



these cells, which in the later development will assume the 

 vegetative functions of the organism. 



In many ova the processes just described are modified to a greater or 

 less extent, but from the frequency of their occurrence they must be re- 

 garded as fundamental and the modifications as secondary. 



Ova which contain but little yolk usually follow more or less closely 

 the typical processes, but where the yolk is abundant, being an inert sub- 

 stance, it acts as a drag upon the protoplasmic activity and produces modi- 

 fication of the segmentation-processes. Two methods of arrangement of 

 the. yolk may be recognized : (a) it may be aggregated more or less com- 

 pletely at one pole of the ovum, such ova being termed telolecithal, or (6) 

 it may be distributed in the meshes of a protoplasmic network, a small 

 quantity of yolkless protoplasm being concentrated around the nucleus of 

 the ovum, while another portion of it forms a thin peripheral layer sur- 

 rounding the yolk, this arrangement being termed centrolecithal. 



In telolecithal ova the third segmentation-division results in the forma- 

 tion of four cells containing very little yolk at one pole of the ovum, while 

 nearly all the yolk is concentrated in the four cells at the other pole (Fig. 

 34, B). This arrangement, which occurs in many MoUusca, constitutes 

 what is termed a total irregular segmentation, in which, owing to the large 

 size of the yolk-containing vegetative cells, the blastocoel is usually com- 

 paratively small. In the Squids the amount of yolk present at the vegetative 

 pole is very great and the protoplasm of the ovum collects upon its surface, 

 there undergoing division and producing a plate of cells, the blastoderm, 

 which by further division gradually extends and finally encloses the inert 

 yolk. This partial segmentation is the result of the presence of a very 

 large quantity of yolk and its telolecithal arrangement, and necessarily 

 obscures greatly the blastula stage. 



In centrolecithal ova which occur in Crustacea and Insects, the division 

 of the nucleus is accompanied by a division of the central yolkless proto- 

 plasm only, the yolk-containing reticulum and the peripheral layer not tak- 

 ing part in the process. As the divisions continue the nuclei gradually 

 approach the surface and finally come to lie in the peripheral protoplasm, 

 which then takes part in the division, a greater or less portion of the inert 

 undivided yolk occupying the blastocoel of the resulting blastula. Many 

 intermediate gradations occur between such a typical centrolecithal and a 

 total regular segmentation, from which both the centrolecithal and telo- 

 lecithal methods are to be derived. 



The blastula is a single layer of cells surrounding a large 

 blastocoel in typical cases, and is a stage quickly passed over 

 in the Metazoa. It is succeeded by a stage in which the em- 

 bryo consists of a double-walled sac open at one end, the gas- 

 trula (Fig. 25). This is most frequently produced from the 



