SPONGES, FLAT WORMS, AND ROUND WORMS 149 



The sexual reproductive cells lie in the middle layer of the body 

 wall. Both eggs and sperms occur in a single individual i.e. 

 Grantia is monoecious or hermaphroditic. Spermato genesis is 

 probably similar to that of an allied genus, Sycon. The pri- 

 mordial germ cell, called a spermatogonium, divides, producing a 

 covering cell, the spermatocyst, and a central sperm mother cell, 

 the spermatocyte. The latter forms a number of spermatids 

 by mitosis; these transform into spermatozoa. The ova arise 

 by the growth of certain cells of the middle layer which are nour- 

 ished by neighboring cells. In Sycon, two polar bodies are 

 formed; the sperm penetrates the egg just before the formation 

 of the second polar body. 



Embryology. The development of the fertilized egg has been 

 observed in Sycon (Fig. 74) and is probably similar to what occurs 

 in Grantia. The egg (a) segments by three vertical divisions 

 into a pyramidal plate of eight cells (b, c). A horizontal division 

 now cuts off a small cell from the top of each of the eight, the 

 result being a layer of eight large cells crowned by a layer of eight 

 small cells. The cells now become arranged about a central 

 cavity, producing a blastula-like sphere (d). The small cells 

 multiply rapidly and develop flagella, while the large cells become 

 granular. The small cells are now partially grown over by the 

 others, forming a structure called the amphiblastula (e). The 

 mass of cells then becomes disk-shaped by the pushing in of the 

 flagellated cells (/). Two layers are thus formed, between which 

 the middle layer arises. The invaginated side soon becomes at- 

 tached (g), and the embryo lengthens into a cylinder at the distal 

 end of which a cavity, the osculum, appears (ti). In the mean- 

 time, spicules and the canal system arise in the body wall. 



SPONGES IN GENERAL 



Grantia has served as a type of the Phylum Porifera, but other 

 sponges vary so markedly from this type, that a general survey 

 of the entire group must be taken before their resemblances to 



