6 DE. J. P. GEMMILL ON THE DEVELOPMENT OF 



This is not the only instance in which a perihsemal sinus-cavity divides into 

 branches which lose themselves within the mesoderm. I find in the adult Solaster 

 that there is a branch extending aborally into each interbrachial septum from the 

 corresponding segment of the external oral circular sinus. This branch soon divides 

 into slit-like spaces, which cease to be recognizable when traced further upwards 

 in the septum (8 a). 



In development the branch in question is not difficult to follow (p. 36), and indeed 

 I was first led to look for it in the adult from having noted it in a specimen of the 

 age of four months. 



Contents of the Egg-Tubes. — The egg-tubes contain : (1) ova in various stages of 

 growth ; (2) follicle-cells, both covering the ova and lining such parts of the gonad- 

 wall as are free from ova ; (3) irregular cells, free in the lumen of the tube, mostly 

 rounded and loaded with fat-granules, but sometimes smaller and resembling leuco- 

 cytes in form. 



Ova. — These rest directly against the inner wall of the egg-tubes without the 

 intervention of follicle-cells. Quite young ova may occur anywhere along the length 

 of the egg-tubes, but as a rule the youngest are most abundant towards the free ends 

 of the tubes. The nucleus or germinal vesicle contains a single large nucleolus, and 

 exhibits in general the characters which are usually found in the nuclei of highly 

 yolked ova. That is to say, it is large from an early stage, and it increases with the 

 growth of the egg ; its membrane is definite and resistant ; there is a delicate 

 achromatic reticulum ; the chromatin is diffuse and difficult to stain ; and the fluid 

 nucleoplasm is abundant and coagulates in shreds. 



The cell-contents stain readily with hsematoxylin, are finely punctate in character, 

 and have a rich orange colour which dissolves out readily in alcohol and other 

 reagents. These facts indicate the early presence of deutoplasmic granules ; but 

 this first deutoplasm is free from ordinary fatty yolk, being quite unaffected by 

 treatment with osmic acid. Indeed, the growth of the ovum may be divided 

 into two periods: the first up till the earliest appearance of the yolk-granules 

 which are fatty in character, and the second till full size is reached and the egg 

 is ready to undergo the changes of maturation. At the end of the first stage the 

 egg has reached a diameter of '15 mm., a third of this measurement being occupied 

 by the nucleus. 



Fatty yolk-granules now begin to gather closely round the nucleus, forming a 

 zone which is at first thin and loose, but afterwards becomes thicker and more crowded, 

 the additions taking place on the outside of the zone (PI. III. fig. 35). Throughout this 

 zone only a small amount of protoplasm remains, forming a reticulum between the 

 granules. Outside of the zone in question there is a layer of protoplasm gradually 

 increasing in size during the active growth of the egg, but gradually also being 



