ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 263 



an annual sponge. According to Orton, there arc two breeding seasons 

 at Plymouth. In June embryos are discharged from large specimens 

 (which subsequently disintegrate). These embryos develop into indi- 

 viduals which, while still very small, produce numerous embryos in 

 October. 



There can be no doubt that the tissues of sponges are much less 

 definite and less permanent than those of typical Metazoa. The in- 

 dividual cells often exhibit a remarkable power of changing their 

 relative positions and also a high degree of polymorphism. Any of the 

 constituent cells of Gremlin compressa may become amoeboid and wander 

 off to some new situation. Collared or pavement epithelial cells may 

 become amoeboid. 



The primary oogonia appear to arise from collared cells, and to 

 migrate from that layer into the mesoglcea. The collar and flagelmni 

 completely disappear and pseudopodia are put out. Before entering 

 upon mitosis they are seen as rounded cells with slightly granular 

 cytoplasm, sometimes with a residue of reserve material in the form of 

 polygonal granules, with a faintly reticulate nucleus, a very large and 

 conspicuous nucleolus, and a thin nuclear membrane. 



The oogonia exhibit mitosis, and during the progress of this they 

 migrate, re-entering the layer of collared cells into the flagellate 

 chambers, where the cell-division actually takes place. Before the 

 oogonia of this second generation are formed the nucleolus is cast out. 

 The details of the division are described. It seems almost certain 

 that another oogonial division takes place in the flagellate chambers 

 very shortly after the first one. Small oocytes result. 



The oocytes have a period of growth in the flagellate chambers and 

 a second in the mesogloea between the chambers. In both there is a 

 marked formation of chromidia or " yolk-nuclei " by extrusion of 

 matter from the nucleolus. It is very probable that shortly after 

 leaving the flagellate chambers and undergoing the process of chromi- 

 dium-formation, the young oocyte exhibits the prophases of a mitosis 

 which really belongs to the first maturation division. The young 

 oocyte puts out long branching pseudopodia by means of which it 

 attaches itself to the mesogloeal surface of the layer of collared cells. 

 These pseudopodia may have a merely anchoring function, but they 

 probably extract nutriment. 



The oocyte continues to increase in size and the nucleus grows more 

 rapidly than the cytoplasm." There is a notable " contraction stage," 

 the cell rounding itself off. Thereafter feeding by means of nurse-cells 

 begins, and after the oocyte has increased greatly long root-like pseudo- 

 podia are put out again. When it reaches a diameter of about ' 045 mm., 

 the oocyte withdraws its pseudopodia and a large first polar body formed 

 in a typical way. The number of chromosomes in each daughter group 

 of the first maturation spindle seemed to be about eight or ten. The 

 second polar body was not found, nor any hint of a reducing division. 



The nurse-cells capture nutrient cells and bring them to the oocyte. 

 The nurse-cells are apparently derived from small amcebocytes which 

 occur scattered in the mesogloea. Other amcebocytes feed voraciously 

 on their own account, even entering the flagellate chambers and 



