278 Transaclions of the Socictij. 



spermatozoon of Grantia was tilifomn and flagellate like the 

 majority of metazoon sperms. 



Tills year more material was sent from Plymouth by Dr. Allen, 

 and one missing stage was found ; this stage nearly completes the 

 story of the fertilization of Grantia compressa, and confirms tlie 

 views expressed in tiie previous paper [S). 



In PI. V, fig. 2, is drawn a part of an oocyte of Gfantia, 

 and on the left a row of choanocytes lining the iiagellated cavity. 

 At NNS is the sperm-carrying collar cell whose nucleus has 

 become altered under the influence of the sperm at MP ; the 

 spermatozoon is ovoid in sliape, and at this period still lies within 

 the cytoplasm of the collar cell. 



Earlier stages than this were not found in the material prepared 

 last year, but in fig. 1 is a much earlier stage. The coarsely dotted 

 area above represents the ectoplasm of the Grantia oocyte (E in 

 PL V, fig. 2), and three collar cells are represented ; the middle 

 one contains at X, MP the head of middle-piece of a spermatozoon. 

 The latter is still elongated, and it seems sufficiently clear that the 

 sperm here beginning to alter in shape was filiform. Such a stage 

 closely resembles what one finds in a typical metazoon fertilization 

 just after the filiform sperm has entered the Qgg. 



EXPLANATION OF PLATE V. 



Lettering. — BM., basal merabrane ; CN., collar cell (choanocyte) nucleus; 

 E., ectoplasm ; EM., egg membrane ; EN., endoplasm ; FL., flagellum ; G., mito- 

 chondrial (in some cases yolk) granule ; H., halo of dense cytoplasm around 

 nucleus ; MP., sperm nucleus ; N., nucleolus ; NU., nucleus ; NNS., nucleus of 

 the sperm-carrying collar cell ; X., extruded nucleolar material still lightly staining. 



All figures on Plate V, excepting 1 and 2, drawn from Champy-Kull material 

 stained after sectioning by Benda's alizarin and crystal violet method. Figs. 

 1 and 2 are drawn from ordinary Champy-Kull material. 



The scale of figures is near fig. 10. 



All Grantia covipressa. 



Fig. 1. — Three collar cells, showing the spermatozoon embedded in one; the 

 head of the sperm is at N, the middle-piece at IMP. The dotted area on the right 

 is the ectoplasm of the oocyte. The relationship of these parts can be seen by 

 examining fig. 2, where E is the ectoplasm. 



Fig. 2. — Later stage, showing at NNS the changed nucleus (NNS) of the collar 

 cell which contains the spermatozoon (MP). This changed collar cell has sunk 

 below its fellows in the germinal epithelium ; on the right is a part of a ripe 

 oocyte, at E and EN are the ectoplasm and endoplasm respectively of the oocyte, ■ 

 and at NU a part of its nucleus. 



Fig. 3. — Collar cell. Arrow points to inhalent chamber. 



Fig. 4. — Progerminative collar cell, about to become an oogonium. 



Fig. 5. — Later stage of above, sliowing enlargement of nucleolus. 



Fig. 6. — Oogonium at the stage of extrusion of nucleolar fragments (N). A 

 halo of thickened cytoplasm indicates perinuclear activity (H). 



Figs. 7, 8. — Stages in extrusion of nucleolar (plastin) fragments. 



Fig. 9. — Nucleuw reformed after its collapse (e.g. in fig. 8). 



Fig. 10. — Another form of extrusion of nucleolar fragments. 



