DEVELOPMENT. 125 



two elements of the genital gland is such that fecundation occurs 

 partially before exclusion, but on account of the inequality in 

 time of development of the sexual elements, the spermatozoa 

 are usually transported b}- the surrounding water and serve to 

 fecundate other individuals. In the dioecious bivalves the 

 spermatic fluid is always conveyed by the water, as there is no 

 act of copulation, and fecundation takes place probably in the 

 pallial or branchial cavity of the females." 



I have recorded the above observations of Dr. Fischer, which 

 agree with the current belief upon the subject of sex in the 

 lamellibranchiates, but the recent observations of Brooks * and 

 Ryder upon the development of the American oyster, show that 

 in that species at least, the sexes are always distinct. Mr. Rj^der 

 has established likewise the dioecious nature of Alya arenaria. 



The number of eggs produced by the pelecypods is prodigious, 

 amounting to a million or more, in the European Ostrea, and 

 from ten to sixtj* millions in the American oyster ; from 400,000 

 to 2,000,000 in Anodonta ; perhaps as many as 12,000,000 in 

 Teredo. Notwithstanding these enormous quantities a certain 

 care of the eggs is exercised by Unio, Anodonta and Sphserium, 

 which retain, hatch and protect their young for a period in their 

 exterior branchiae {branchial uterus). Dr. Lea has observed all 

 the four leaves of the branchiae filled with ova in several species 

 of Unio, recalling the incubatory pouch of the marsupial mam- 

 malia."}" 



DEVELOPMENT. 



The researches of John Hunter led him to the conclusion that 

 each stage in the development of the highest animals corre- 

 sponded to the permanent form of some one of the lower orders. 

 Since his time a number of eminent investigators have partially 

 confirmed this genei'alization, and more exactly defined it by a 

 larger induction of facts. Woodward thus states it : 



" In the earliest period of existence all animals display one 

 uniform condition ; but after the first appearance of special 

 development, uniformity is only met with amongst the members 

 of the same primary division, and with each succeeding step it 

 is more and more restricted. From that first step, the members 

 of each primary group assume forms and pass through phases 

 which have no parallels, except in the division to which each 

 belongs. The mammal exhibits no likeness, at any period, to 

 the adult mollusk, the insect or the star-fish ; but onlj^ to the 

 ovarian stage of the invertebrata, and to more advanced stages 



* Johns Hopkins Laboratory Studies, 1, pt. 4, 



f Raymond has observed that an Algerian flnviatile gastropod {Melania 

 tubereulata), carries its younq; in the branchial cavity, and that these are 

 accustomed to quit and re-enter this temporary lodging. 



