IV. i INITIAL STRUCTURE OF THE GERM 183 



6. ECHINODERMATA. 



Easy to obtain in large numbers and eminently suitable for 

 experiment, the eggs of the sea-urchins and starfishes have pro- 

 vided the analytical embryologists with abundance of material; 

 and it is in this group that the possibility of rearing a perfect 

 larva from a single blastomere was first demonstrated by the 

 classical researches of Driesch. 



The experiments which have been carried out, mainly by 

 Driesch, but also subsequently by others, are very numerous and 

 fall into two principal classes. 



In the first the development of parts whether isolated blasto- 

 meres or groups of blastomeres, fragments of unsegmented eggs 

 fertilized or unfertilized, or pieces of blastulae and gastrulae has 

 been observed, the rate of their development recorded, and the 

 relation of the number of cells and dimensions of the partial 

 larvae to their germinal value determined. By the second series 

 of investigations the effect of alteration of the type of nuclear or 

 cell-division and consequent displacement of the blastomeres upon 

 the subsequent development has been discovered. 



Before discussing these experiments, however, a word must be 

 said as to the normal behaviour of the whole egg. 



The structure, segmentation and formation of the primary 

 germinal layers have been very completely studied by Boveri in 

 the sea-urchin, Strong ylocenirotus lividus (Fig. 92). The axis of the 

 ovarian egg is marked by the excentric position of the nucleus ; 

 at the point on the surface nearest which it lies the polar bodies 

 are formed, and this point is the animal pole. At this point 

 there is a fine canal (micropyle) in the jelly which surrounds 

 the egg. After the extrusion of the polar bodies a brown pigment 

 which had previously been uniformly distributed through the 

 cytoplasm becomes aggregated in a dense but quite superficial 

 subequatorial zone, the upper somewhat ill-defined border of 

 which may, however, extend into the animal hemisphere. The 

 pigment-free region left about the vegetative pole occupies from 

 ^ to ^Q the volume of the whole ovum. The spermatozoon may, 

 but need not, approach the egg by the micropyle. The fertiliza- 

 tion spindle lies in a plane parallel to the equator of the egg (the 



