MUSEUM OF COMPARATIVE ZOOLOGY. 41 



and it at once becomes evident that the spaces supposed to be canals 

 are for the most part minute spheroidal cavities or vacuoles. Usually 

 there is only a single vacuole in a villus, although occasionally two are 

 to be seen in the same cross section (Plate II. Fig. 4). Not all of the 

 villi contain these cavities. Taking into the account their abundance on 

 successive sections from the villous layer, I should estimate that not 

 more than one half or three quarters of them present this feature. The 

 proportion to be seen upon a single section is, of course, much less than 

 this. They are most abundant in the stalks, but occasionally one is 

 also seen in the head. Upon the egg, where they were found most 

 abundantly, they were rather more numerous in the micropylar region 

 than at the opposite pole. In the latter region there were, however, 

 sometimes as many as three or four in one villus, although the size (0.5 

 to 1 /a) was the same as at the micropylar pole. I have in a few cases 

 observed that the vacuoles were elongated, and then they were always 

 of uniform calibre and were curved. Occasionally (Plate II. Fig. 4) such 

 a tubular vacuole appears to communicate at one end with the inter- 

 villous spaces. Concerning the nature of the contents of these vacuoles, 

 I can only say that they do not stain, and do not appear differently 

 from what one would expect if they were cavities simply filled with the 

 mounting medium. 



The differences between the membranes in mature ovarian eggs and 

 those recently deposited are principally the result of the swelling of the 

 layers by the water, and do not require any further explanation. 



The foregoing account of the zona radiata in Lepidosteus contains 

 descriptions of two features which appear to me to bear directly on the 

 condition of the zona radiata of fishes in general. 



First. The proof that the striate appearance of the zona is due to 

 pore-canals, although very generally assented to by the most competent 

 observers, especially in recent years, has nevertheless hitherto rested 

 upon comparatively slight evidence. That this evidence has been 

 meagre depends upon the excessive minuteness of the structures in 

 question. It is not to be overlooked, in the first place, that the tubu- 

 lar nature of the pore-canals in the case of the perch, as originally de- 

 scribed by Johannes Miiller ('54) for what he called the "Eikapsel," 

 has not the slightest bearing upon the nature of the pore-canals of the 

 zona radiata, since the egg capsule of Miiller is a structure entirely 

 different from the zona. I cannot, however, avoid the conviction that 

 his opinion as to the tubulated condition of that capsule has had con- 



