xm EARLY DEVELOPMENT IX VARIOUS TYPES 581 



reserve of yolk. It then passes through a complicated 

 series of larval stages, gradually leading up to the adult 

 form. 



Early Development of other Types. The presence of 

 a greater amount of food-material in the egg renders it 

 possible for the embryo to go on developing further 

 than the gastrula-stage before being hatched, and, as a 

 general rule, the greater the relative quantity of yolk 

 present in the ovum of an animal, the less clearly can a 

 gastrula-stage be recognised. 



In the earthworm and mussel, it will be remembered (pp. 

 349 and 410), the segmentation is entire, but unequal, and 

 the larger lower cells become invaginated to form the endo- 

 derm and enteron while the smaller upper cells give rise to 

 the ectoderm. In the earthworm the blastopore does not 

 become closed, but gives rise to the mouth. 



In the frog (p. 201) the enteron arises by a split 

 appearing amongst the yolk-cells, beginning at the 

 edge of the blastopore and gradually extending inwards : 

 the process is supplemented by an invagination of the 

 ectoderm. The enteron is at first a very narrow cleft, 

 but soon widens considerably (Fig. 64, I, ent) : for 

 some time it does not actually communicate with the 

 exterior, the blastopore (Up) being filled up by a yolk- 

 plug (yk. pi]. As the enteron extends forwards, and 

 the relatively small segmentation-cavity (bl. ccel) gradu- 

 ally disappears, the edges of the lower margin of the 

 blastopore approach one another and, uniting in the 

 median plane, give rise to a vertical, grooved streak 

 the primitive streak and groove, as it is called. 



In the centrolecithal egg of the crayfish (Fig. 98) a gas- 

 trula-stage is formed by invagination, but as the centre of 

 the oosperm is filled with solid yolk in the place of a seg- 

 mentation-cavity containing fluid, the invagination only 

 extends a short distance inwards, the enteron being rela- 



