382 DR ARTHUR T. MASTERMAN ON THE 



compressed, but is about half as long and twice as broad as an epiblast cell. The 

 nuclei are aggregated towards the inner end centripetally, and the cytoplasm lying- 

 inside them is singularly free from yolk-globules. On the other hand, the part of the 

 cell lying outside the nucleus is filled with very large yolk-granules, most of them 

 exceeding in size those found elsewhere. The contrast between the outer and inner 

 portions of these cells is in consequence very striking, and in all later stages the 

 mesenteron and to a lesser extent the ccelom have their inner limits clearly indicated by 

 their darker appearance. The cell-walls are like those of the epiblast, but naturally 

 there is a basal wall at each end separating the hypoblast from the mesenchyme distally 

 and from the hypenchyme proximally. Each of these consists of a mesh work of cyto- 

 plasm, through which are scattered numerous nuclei, which are usually slightly smaller 

 than those of the epiblast or hypoblast. Yolk-globules are numerous and arranged 

 indiscriminately. As a rule, they are smaller than those of the hypoblast, and average 

 about the same as those of the epiblast. The mesenchyme completely fills the archiccele 

 cavity, and the hypenchyme fills the archenteron to the very lips of the blastopore. 

 The hypoblast walls are constricted below the ' head ' and lie in contact, but this 

 condition is not universal. The ' head ' portion is probably the anterior ccelom of the 

 next stage, and the constricted part is the mesenteron. Such is the last embryonic stage 

 of Cribrella, which, although solid itself, has been produced from a solid morula by a 

 normal process of invagination. 



Part II. Larval Period. 



The larval period lasts from the hatching epoch to the time when the larva fixes itself 

 permanently, in the examples which are freed from the brood-cavity. The exact 

 duration of this larval life, as already remarked, is hard to determine, but it is probably 

 on the average about seven to nine days. 



I have, for comparison with other species such as Asterina, and in order to facilitate 

 description, divided this larval period into four stages, clearly recognised by the external 

 appearance. 



External Structure. (Stages A-D.) 



It may be well to glance at these four stages externally before proceeding to the 

 internal structure of each. 



Stage A — (fig. 20). — The newly hatched larva is nearly spherical, though the main 

 antero-posterior axis is distinctly elongated and the anterior end is somewhat pointed. 

 At the blunter posterior end there can still be discerned a small epiblastic pit marking 

 the final closure of the blastopore, though the pit is blind at its inner end. This 

 blastoporic pit is slightly inclined towards the ventral surface. The larva is uniformly 

 ciliated, and moves gently forwards through the water when set free from the brood- 

 cavity. It invariably moves with the anterior or tapering end forwards. The larva, like 





