PORIFERA. 143 



epiblast cells, now become amceboid. 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 gastrula cavity there arises a hyaline structureless layer, 

 which is more closely attached to the epiblast than to the hypo- 

 blast, and is probably 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 Metschnikoff's observations 

 (No. 134) to be a 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 disappear- 

 ance is nearly coincident with the complete obliteration of the 

 blastopore, an event which takes place shortly after the attach- 

 ment of the larva. 



Not long after the closure of the blastopore, calcareous 

 spicules make their appearance in the larva as delicate un- 

 branched 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 inhalent 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 constant for the different larvae. On the central gastrula 

 cavity of the sponge becoming placed in communication with 

 the external water, the hypoblast cells lining it become ciliated 



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. 



