XVI 



ECHINODERMATA 



507 



once and then stops, whilst each of the larger micromeres divides three 

 times, giving eight cells. Each macroniere and each uiesomere 

 divides tive times, giving in each case sixty-four cells. 



When these divisions have been accomplished each cell acquires 

 a cilium, and the hollow sphere of cells begins to rotate within the 

 egg-membrane, which it soon bursts. It then rises to the surface of 

 the water as a free-swimming blastula. 



If the description of the cleavage of the egg, which has just been 

 given, has been correctly followed, it will be seen that the blastula con- 

 sists of 4 x 2 (smaller micromeres) + 4x8 (larger micromeres) + 4 x 64 

 (macromeres) + 8 x 64 (rnesomeres) cells, i.e. SOS cells altogether. 

 The blastula soon becomes flattened at the vegetative pole, and the 

 cells here begin to divide and to give off primary mesenchyme into 

 the blastocoele, just as happens in the case of the larva of Ophiotlirix 



mesm 



macm 



mi cm 



FIG. 383. Two stages in the segmentation of the egg of Strongylocentrotus Jicidtis. (The 

 segmentation of the egg of Echinus esndcntus pursues an identical course.) (After 

 Boveri. ) 



A, 10-cell stage. B, 3^-cdl stage, mucnt, macroim-re ; inesm, mi'somere ; 



i, micromere. 



fragilis. This mesenehyme originates, almost certainly, from the 

 descendants of the smaller micromeres. Only a limited number 

 (fifty or thereabouts) of these mesenchyme cells are formed ; they 

 arrange themselves in a ring round the periphery of the blastocoele, 

 the plane of which is parallel to the plane formed by the vegetative 

 surface. 



In two places, diametrically opposite to each other, the ring is 

 thickened and consists of a heap of cells ; elsewhere it consists of a 

 single line of them connected with each other by pseudopodia. In 

 the centre of each heap a little triradiate calcareous spicule appears; 

 these heaps of cells and their contained spicules are the first organs 

 which indicate the future bilateral symmetry of the larva. 



Then the blastula becomes a gastrula by the invagiuation of the 

 archenteron, which begins in the centre of the flattened surface. As 

 in Ophiothrix and Asterias the archenteron is a narrow tube of small 

 diameter compared with the gastrula, but it reaches nearly to the 



