Pseudogastrula Stage in Calcareous Sponges. 99 



By this time the granular cells have increased so much in 

 bulk, and became so far invaginated as to reduce the 

 segmentation cavity to a mere slit, in which, however, the 

 primitive mesoblastic tissue is still recognisable (Fig. -i.) 



The next distinct stage is represented in Fig. 5. The 

 embryo is now solid and almost spherical. The columnar 

 ■cells have elongated and their inner ends reach nearly to 

 the centre of the embryo. The segmentation cavity is 

 perhaps represented by a dark area in the centre. The 

 granular cells form a hemispherical mass, which is the 

 posterior half of the embryo. This mass has become 

 difterentiated into two distinct parts — (1) an external single 

 layer of clearer, more or less cubical, nucleated cells ; and (2) 

 an internal mass of highly granular, larger, nucleated cells, 

 which are ovoid or more or less polygonal from mutual pressure. 



In the latest stage which I have seen (still within a 

 flagellated chamber), the embryo has become somewhat 

 pointed at the anterior extremity, and the boundaries between 

 the internal granular cells are no longer distinctly visible. 

 Fig. 6 is a diagram of this stage, representing the free 

 swimming embryo as it leaves the parent sponge on its 

 way to seek a place of fixation. As already stated, I have 

 not myself seen the cilia, but there cannot be the slightest 

 doubt, after the observations of so many authors on the 

 living organism, of their existence. 



Certain observations of Metschnikoff on Sycandra form a 

 strong confirmation of my views as to the development of 

 the Sycon type of calcisponge. This author* states that in 

 the older larvte, the posterior part, devoid of cilia, does not 

 remain so simple as in the earlier stages. A cavity is 

 developed in it which divides it into two layers. Sometimes 

 also he found and figures {loc. cit. Fig. 11) a larva which 

 closely agrees with that represented in my Fig. 5, in which 

 the posterior half consists of an outer layer of epithelial cells 

 and an inner mass of rounded cells closely packed. This 

 internal mass he derives from the inner of the two layers 

 into which the granular cells are first of all divided. Exactly 

 how the original division of the granular cells took place is 

 not made clear. Metschnikoff appears to have observed the 

 fact that there is a division only after the pseudogastrula 

 stage has been passed through. I suspect that the true 

 course of events is very much the same as I have described 



* " Zeitschrift fiir wissensch. Zoologie," Vol. 32, p. 368 et seq. 



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