276 ME. G. brook: ok the 



one membrane (the %ona radiatd) in tlie Spanisli Mackerel, as 

 also Messrs. Kiugsley and Conn in tlie Gunner. The outer 

 membrane may indeed be only an outer layer of the zona radiata ; 

 but it is easily separated and made prominent by the use of 

 reagents. Balfour (' Comp. Embryol.' i. p. 50) says that " in 

 osseous fishes the vitelline membrane is usually either absent, or 

 may perhaps in some instances, e. g. in the Perch, be imperfectly 

 represented. In the ripe ovum of the Herring there is a distinctly 

 developed membrane exterior to the zona radiata, which is pro- 

 bably the vitelline membrane." A vitelline membrane does not, 

 however, appear to have been usually recognized in pelagic 

 teleostean eggs. 



The oil-globules scattered over the upper hemisphere are 

 situated inwardly to the zona radiata, and sink into little pockets 

 pushed into the yolk from its surface. These two membranes 

 may also be distinctly seen in the unfertilized egg ready for 

 extrusion taken from the body of the fish ; but they are then in 

 a relaxed and shrivelled condition. After the closure of the 

 blastoderm at the caudal end of the embryo, the yolk becomes 

 invested by another membrane, which is termed by Eyder the 

 "epiblastic sac," the origin and development of which will be 

 discussed in its proper place. 



The zona radiata appears to become thinner as development 

 proceeds. 



Micropyle. — I have several times seen the circular micropyle 

 opening in the zona radiata. It seems to be surrounded by a 

 depression on the outer surface of that membrane, causing a 

 slight protuberance on the inner surface. 



Sect. 1. Segmentation Stage to Formation of Blastodermic Rim. 

 9 A.M. In the newly laid ovum the germinal disk is not distin- 

 guishable until the first furrow begins to form, faint at first, 

 then forked at each end (fig. 1) ; and the forking may be seen 

 gradually creeping round until the first two cells are formed, as 

 shown in fig. 2. This takes about an hour to accomplish ; and the 

 first furrow is then very thick and distinct, the outline of the cells 

 gradually shading off" as it recedes from the furrow until on the 

 opposite side it is only barely visible. Somewhat later this out- 

 line becomes more sharply defined. When first formed the two 

 cells of the disk are about "35 millim. in diameter each ; but 

 these increase in size to '41 millim. before any further subdivision 



