Pseiulogadrida Stage in Calcareous Sponges. 95 



layer, which is more closely attached to the epiblast than to 

 "the h3^poblast, and is probably derived from the former. 

 There wonld seem according to MetschnikofF's 

 observations 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." * 



The principal changes which take place in the develop- 

 ment of the tixed larva into the j'oung sponge are the 

 development of spicules in the mesoblast, the perforation of 

 the double wall of the gastrula by the osculum and pores, 

 and the conversion of the hypoblast cells into the collared 

 cells so characteristic of sponges. 



Such, then, is the generally accepted history of the 

 development of Sycandra, given as briefly as possible. 

 Perhaps the most remarkable feature in its whole course is 

 the Pseudogastrula. Although Balfour states that no 

 importance can be attached to this phase of the life-history, 

 it is obvious that a stage of such constant occurrence 

 amongst the Sycons, and found also, according to Keller,-]- in 

 the Leucons, cannot be entirely meaningless. Sollas, indeed, 

 has made a speculative attempt to explain it on purely 

 theoretical grounds. "We may conjecture," says he, "that 

 the larva which becomes a sponge now, by invagination of 

 the ciliated layer, is a descendent of a form which used to 

 Ijecome a coral by the invagination of the other layer, that 

 is, that a form on the way to become a coelenterate, took 

 the wrong turn for once, and so ended in a cul-de-sac, and 

 became a sponge. Thus the abnormal kind of invagination 

 in Sycandra may be an instance of what is termed ' reversion 

 to an ancestral type ;' on the other hand it may simply 

 indicate the balancing play of forces on the young organism, 

 so that it looks as if it could not make up its mind, and was 

 undecided as to whether to turn the flagellated layer inside 

 and become a sponge, or outside, and become a coelenterate. 

 Between these alternative possibilities, we cannot decide." | 

 All this is mere hypothesis, and I venture to hope that the 

 correct explanation of the Pseudogastrula stage may be 

 found in the following observations. 



• Balfour, loc. cit. 



+ " Uutersuch. iiber die Anat. uud Entw. einiger Spongien des Mittel- 

 meeres." Unfortanately, I am unable at present to obtain access to this 

 work. I cite it upon the authority of Met?chuikoff. 



+ "The Structui-e and Life-History of a Sponge." Proceedings of the 

 Bristol Naturalists' Society., Vol. 3, 1880. 



