PO RIFE HA. 



117 



ec 



which, together with the other epiblast cells, now become amoeboid. 

 They become at the 

 same time clearer and 

 permit a view of the 

 interior of the gastrula. 

 Between the epiblast 

 cells and the hypoblast 

 cells which line the gas- 

 trula cavity there arises 

 a hyaline structureless 

 layer, which is more 

 closely attached to the 



'X^J 



epiblast than to the 

 hypoblast, and is pro- 

 bably derived from the 

 former. A view of the 

 gastrula stage after the 

 larva has become fixed 

 is given in fig. 66. 



There would seem 

 according to Metsch- 

 nikoff's observations 

 (No. 134) to be 



a 



FIG. 66. FIXED GASTRULA STAGE OF SYCANDRA RAPH- 

 ANUS. (Copied from Schulze.) 



The figure shews the amoeboid epiblast cells (ec.) 

 derived from the granular cells of the earlier stage, 

 and the columnar hypoblast cells, lining the gastrula 

 cavity, derived from the ciliated cells of the earlier 

 stage. The larva is fixed by the amoeboid cells on the 

 side on which the blastopore is situated. 



number of mesoblast cells interposed between the two primary layers, 

 which he derives from the inner part of the mass of granular cells. 



After invagination the cilia of the hypoblast cells can no longer 

 be seen, and are probably absorbed; and their disappearance is nearly 

 coincident with the complete obliteration of the blastopore, an event 

 which takes place shortly after the attachment of the larva. 



Not long after the closure of the blastopore, calcareous spicules 

 make their appearance in the larva as delicate unbranched rods 

 pointed at both extremities. They appear to be formed on the 

 mesoblast cells situated between the epiblast and hypoblast 1 . The 

 larva when once fixed rapidly grows in length and assumes a 

 cylindrical form (fig. 67 A). The sides of the cylinder are beset 

 with calcareous spicules which project beyond the surface, and, in 

 addition to the unbranched forms, spicules are developed with three 

 and four rays as well as some with a blunt extremity and serrated 

 edge. The extremity of the cylinder opposite the attached surface is 

 flattened, and, though surrounded by a ring of four-rayed spicules, is 

 itself free from them. At this extremity a small perforation is 

 formed leading into the gastric cavity, which rapidly increases in size 

 and forms an exhalent osculum (os.). A series of iuhalent apertures 

 is also formed at the sides of the cylinder. The relative times of 

 appearance of the single osculum and the smaller apertures are not 



1 Metschnikoff was the first to give this account of the development of the spicules 

 in Sycandra, but Prof. Schulze has informed me by letter that he has arrived at the 

 same result. 



