Pseudogastrula Stage in Calcareous Sponges. 93 



undeservedly fallen, and that it ma}^ lead to a stricter 

 supervision of the instruments employed in nautical 

 astronomy. I believe that all gun barrels have to be 

 submitted to a Government test, but if one burst, little harm 

 would be done, except to the user. In the case of sextants 

 and chronometers no supervision is exercised in their 

 manufacture, and the selection is left to individual caprice, 

 yet a faulty one may cause the loss of much property and 

 many lives. 



Art. XIV. — On the Pseiidogastrida Stage in the Develop- 

 ment of Calcareous Sponges. 



By Arthur Dendy, M. Sc, F.L.S. 



Fellow of Queen's College, University of Melbourne. 



(With Plate lA.) 



[Read November li, 1889.] 



Thanks to the researches principally of MetschnikofT, 

 Schulze and Barrois, we are now in possession of a 

 tolerably full and accurate account of the development of 

 the Sycou type of calcareous sponges, as represented by the 

 genus Sycandra. It is in the hope of contributing a small 

 addition to our knowledge in this department of embryology 

 that the present paper is written. 



Before going on to describe my own ol:>servations, it will 

 be advisable to give a brief account of the now generally 

 accepted views concerning the history of the development 

 of Sycandra — such, for example, as is to be found in 

 Balfour's " Treatise on Comparative Embryology." 



The ovum is a naked, amoeboid, nucleated mass of 

 protoplasm, which, after fertilization, undergoes the early 

 stages of its development within the tissues of the mother 

 sponge. The ovum first divides vertically into two and 

 then into four segments. The next two divisions are also 



