THE STAEFISH 80LASTEE ENDEOA. 13 



the widening and deepening of a large central furrow in the under pole. Accordingly 

 at first the blastoporic opening is often slit-like in shape. The final smoothing of the 

 surface- wrinkles advances with the progress of gastrulation, the opening out of surface 

 provided by the former process compensating in part for that portion of the blasto- 

 derm which is invaginated. The blastopore now becomes a large circular opening in 

 the middle of the distinctly fiattened under surface, while the upper aspect of the 

 gastrula is rounded or dome-like in shape. The invagination does not involve the 

 whole of the pale area at the lower pole. In the fully formed gastrula the line of 

 colour-transition is found outside the lip of the blastopore, at about a third of the 

 distance between this and the centre of the dome. 



The movement of the gastrula now becomes one of smooth rotation round a vertical 

 axis, accompanied by slight intermittent horizontal progression, giving a somewhat 

 epicyclic character to the whole. In the specimens under observation, as a rule, the 

 direction of movement was dextral * as observed from above, but reversal took place 

 from time to time, and in the course of a few days there seemed to be longer duration 

 of the sinistral f than of the dextral periods. Probably the changes occur rhythmically 

 in nature, though I had evidence that they were hastened by stimulation. 



In healthy specimens the archenteron (PI. I. fig. 6) is a large cavity lined by columnar 

 ciliated hypoblast and separated from the epiblast by a very narrow space containing 

 stellate mesenchymal cells. The cavity is empty, except for stray yolk-granules and 

 occasional cells (see p. 45). 



In Cribrella, according to Masterman (i8, p. 382), the archenteron becomes com- 

 pletely filled up at this stage by a network of " hypenchyrne " cells which are thrown 

 off from the primitive hypoblast. This does not occur in Solaster, at least not in healthy 

 specimens, but at a slightly later stage a certain number of cells are budded off into 

 the anterior and posterior coeloms of the young larva. In the latter cavity they 

 frequently give rise to a homogeneous spherical body, the appearance of which strongly 

 suggests that it may function as a statolith during the middle and later free-swimming 

 stages (p. 46). 



Chronology. — The following details regarding the progress of the above changes in 

 examples of a set of ova under observation will be of interest : — 



At time of shedding The first maturation-spindle lying close under the upper pole of 



the egg. 



3 hours after shedding . . . Both polar bodies extruded. 



5 „ „ ... The membrane of fertilisation completely separated. 



9 „ „ ... The yolk-free disc at the upper pole becomes oval and then dumb- 



bell in shape (10 hours) ; it next divides into two, while the 

 first cleavage-furrow begins to form (11 hours). 



Same as that of the hands of a clock. 

 Opposite to that of the hands of a clock. 



