ZOOLOGY. oOO 



At a former meeting he had stated that in a single instance he had found 

 one of these cases partially formed in the oviduct, and was struck with the 

 fact that it contained no yolk. Through the kindness of Mr. Green, the 

 Curator of Comparative Anatomy, he had had an opportunity of examining 

 the oviduct of a skate in which an incomplete egg-case existed in each 

 oviduct; two of the horns, and the bundle of threads at their base, and a 

 portion of the body of the case, were already formed, but there were no 

 yolks in the oviduct, and only one corpus luteiim in the ovary, probably con- 

 nected with the previous detachment of an ovum. These observations Avould 

 seem to show that this egg-case is more or less completely formed first, the 

 yolk subsequently introduced and closed in, contrary to the order of things 

 with eggs generally. The materials of the egg-case were detected in the 

 tubules of the gland of this oviduct, and consisted of granules and long 

 slender threads. The case is formed in the centra! cavity of the gland, and, 

 as it is built up, the formed portions gradually extend into the lower part of 

 the oviduct. The ovulation of skates resembles that of birds rather than of 

 ordinary fishes. In the latter, the eggs are all formed simultaneously, and 

 discharged at once, or nearly so, while in the former, as in birds, one yolk 

 descends in the oviduct at a time, is encased in the covering, and lost before 

 another can go through the same process. 



Professor Agassiz said that the communication of the president was of 

 importance, as it bore upon several physiological points now under discus- 

 sion. He had been shown by the president the egg still in the ovary (where 

 it must have been fecundated), and the shell below prepared to receive it. In 

 this connection, he was reminded of a fact noticed by himself some time 

 since, Avith reference to those organs upon the side of the skate called clasp- 

 ers, and which are supposed to be used for prehensile purposes. It occurred 

 to him that they might be organs of copulation ; and he found that when 

 they were rotated forwards and upwards, an opening in them was brought 

 opposite to the spermatic duct, and he imagined that they could be intro- 

 duced readily into the female organs, into the oviducts, and reach the glands 

 described, whence the spermatic fluid would pass up. The president's obser- 

 vations rendered this view of the functions of the clasper still more probable. 

 Plagiostonies have a very different method of reproduction from other fishes. 

 Like birds, they produce few but large eggs, and these are found to be of 

 various sizes and different degrees of development in the ovary, indicating 

 that several years are required for their maturity, as is the case in turtles. 

 These facts and others serve to confirm the affinities of the sharks and skates, 

 and to separate them from fishes proper. Aristotle do3s not speak of P/a- 

 giostomes with fishes, but calls them Selachians, and Professor Agassiz fol- 

 lows the ancient naturalist, giving them the same name. If the Selachians 

 constitute a natural class, then some of the data of palaeontology may be 

 better understood than they now are. Fishes are generally considered to 

 have been the earliest vertebrate animals created; but, in fact, they were not. 

 The earliest were Ganoids and Selachians. 



EMBRYOLOGY OF FISHES. 



The development of Fishes is distinguished from that of the scaly Rep- 

 tiles, the Birds, and Mammals, in that neither amnion nor allantois is formed. 

 In the beginning, that dividing or cleaving of the yolk is perceptible, which 

 we have already spoken of in different classes of invertebrate animals. 



