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Part III. — Twenty -fifth Annual Report 



out, appears to have a distinct cell-wall. The egg is oval and measures 

 about 0'7mm in diameter; it is filled with yolk granules. The nucleus, 

 according to M'Intosh, shows as a cleared part in the midst of the egg. 

 In some cases it is burst clear out of the egg, and can be seen as a clear 

 circular body with a prominent nucleolus. It measures 0'5mm. and less 

 in diameter. 



Seen alongside the mantle of the male, that of one female, with its 

 ovisacs had a very slight cream colouration. 



In the female the ovisacs were more equally developed, inter se, in 

 contrast with the two distinct stages of development in the same mantle 

 of the male. The latter fact will bear upon the question of the duration 

 of the spawning period, and the possibility of the mussel spawning twice 

 during the same season. 



In one mussel which had a thin orange-coloured mantle, I was unable 

 to make out definite sexual elements. 



April 15, 1907. — Some mussels that were taken from the wall of the 

 big pond had been kept in a sunk box. 



On opening them they were found to be fat. The sexes were very 

 distinct. The male had a deep cream-coloured mantle and abdomen. 

 The female was of a chocolate-pink. 



The sperm-sacs were distinct — white. The vessels in the mantle were 

 translucent — amber. On cutting into the mantle a milky fluid issued. The 

 sperms were alive, and, under Zeiss Obj. D.D. : oc. 4, showed fairly large. 



In the female the ovisacs were only distinguished with difficulty in the 

 general ground of the mantle. The vessels were similar to those in the 

 male. On teasing the mantle the eggs were obtained, but in most cases 

 they were burst. They were oval, round, or pear-shaped, measuring 

 •07-*lmm. in diameter. There is a large pore in the zona, and by it the 

 yolk corpuscles pour out. The yolk granules wriggle about ; they are 

 smaller than the sperms. I was not able to make out any connection 

 of the eggs to the ovarian tissue, even when they were sticking 

 together en masse. The pore in the zona is a clean-cut hole. The pres- 

 sure of the cover glass in some cases forces out the nucleus, although it 

 is of greater diameter than the opening. In other cases the nucleus 

 remains inside while the yolk granules nearly all pour out. The nucleus 

 retains its shape after it issues from the pore. It exhibits a slight 

 granular structure. Sometimes there are a few yolk granules attached 

 to it. Occasionally the nucleus is accompanied by what appears to be a 

 little nucleus. 



The gills and labial palps in both sexes are sometimes orange -coloured ; 

 in the female rather darker than in the male. In the latter sex, however, 

 they are also found colourless, 



April 29, 1907. — The mussels supplied at this date were not apparently 

 so far advanced as some already examined. 



Some of them were fat and yellow in colour. On dissecting, a pale 

 white fluid was got ; it is albuminous, and coagulates on the slide when 

 fresh water is added. It contained small round corpuscles, and cells full 

 of dark refractive corpuscles. 



The other mussels were pale and of a slight orange tinge, having thin 

 mantles. In them were observed pitcher-shaped cells showing a clear 

 nucleus. The eggs are not very far developed. 



May 1, 1906. — The mussels on this date were nearly all pink. They 

 were females with well-developed ova. The pink region is the mantle, 

 which is almost wholly that colour. 



May 31, 1907. — Some mussels were covered with barnacles. They 

 were not fat. The mantles were thin, having a clear ground tissue, 

 and reproductive organs nearly ripe. 



