ECHINODERMATA 



393 



Holothurioidea. — The earliest stages of development 

 are the simplest in the Holothurioidea. We follow Selenka's 

 account (No. 54) of the development of Synapta digitata. 



Cleavage is entirely regular. By means of the first 

 division the egg is halved. Since the newly iormed blas- 

 tomeres always divide into equal parts, this process being 

 repeated nine times consecutively, there finally arises a 

 stage the prismatic cells of which are approximately equal 

 in size, and arranged in the form of a hollow sphere. They 

 have already acquired cilia, although the blastula is still 

 enclosed by the vitelline membrane. (A similar stage in 

 Holothtiria is shown in Fig. 180 A.) At this stage the 



Fig. 173.— Blastosphere of Sxjnapta digitata at the beginning of gastrulation, still 

 lying witliin the egg-membrane (after Selenka). 



further division of the blastomeres is suspended for a con- 

 siderable time, only subsequently to proceed slowly at the 

 vegetative pole, and at first at this pole alone. Gastrulation 

 is initiated by means of this cell-proliferation realized at 

 the vegetative pole (Fig. 173). The result is a regular 

 gastrula with a small archenteron (Fig. 174). In this 

 stage the embryo becomes a free-swarraing larva, which 

 moves about by the aid of its long cilia. The gastrula very 

 soon undergoes a change, for the archenteron bends towards 

 the wall of the gastrula, and unites with the ectoderm 



