214 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



embryo lias grown considerably and when it is on the eve of attaching 

 itself permanently, it measures from -g^th down to -^th of an inch in 

 diameter. The mode of fixation of the fry of both species is probably 

 the same, but the mode of incubation — the one in the mother, the other 

 in the open water — we see is widely different, differing as greatly in this 

 respect as do the eggs in size and details of construction, as shown by 

 the measurements which I have given. It must not be forgotten, how- 

 ever, that the material from which I prepared my sections was received 

 from Europe, in January and March, when it is to be supposed that the 

 reproductive organs were not yet fully developed, and that consequently 

 the dimensions of the ovarian ova as found by me are rather to be con- 

 sidered as being below than above their true ones when fully developed 

 at the height of the spawning season. 



It is a very remarkable fact that one finds individual specimens of 

 oysters in which the reproductive organs have undergone total atrophy 

 or wasting away at the completion of the spawning season. Examining 

 sections through the body-mass of spawn-spent oysters taken from their 

 native waters in August last, I find that the whole of the connective 

 tissue subjacent to the mantle, and between the latter and the liver, 

 especially over the sides of the body-mass, has disappeared, together 

 with all traces of the reproductive organs, including the superficial 

 branches of the efferent ducts. At the first bend of the intestine there 

 is still some of the connective tissue remaining; but even here and in 

 the mantle it has changed its character entirely, and become very spongy 

 and areolar, instead of solid, and composed of large vesicular cells, such 

 as are met with when the animal is in a better condition of flesh. In 

 fact, it appears as if this mesenchymal or connective tissue substance 

 had been used up and converted into reproductive bodies — generative 

 products — in the case of the spawn-spent and extremely emaciated indi- 

 viduals. In sections from individuals in various conditions from that in 

 which the rudimentary network of generative tubules has just appeared 

 in the connective tissue, on up to those in which the reproductive tissues 

 are enormously developed in bulk and proportion to the mass of the 

 remaining structures, there is a perfect gradation from their complete 

 absence to their full development. This would appear to be very strong 

 evidence in support of the theory that the reproductive follicles, or 

 tubules, are developed anew each season directly from the specialization 

 of certain strings or strands of connective tissue cells. 



Many animals manifest a periodic development of the glandular por- 

 t ions of the reproductive organs ; but I know of no form in which there 

 is any such presumptive evidence that these organs are annually regen- 

 erated and finally altogether aborted as seems to be the case with the 

 i »yster. Together with the changes here described, the most remarkable 

 changes in the solidity and consistence of the animal take place. The 

 shrinkage of a spawn-spent oyster in alcohol or chomic acid solution 

 is excessive, and will, when complete, reduce the animal to one-tenth of 



