290 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



(Mink observed the eggs being ejected in a sort of stream from the 

 Openings into the water and settling in loose clusters on adjacent objects. 

 Yen-ill and Carpenter have confirmed these observations, and add that 

 the hinder part of the foot is so raised as to form a sort of funnel, out of 

 which the eggs emerge. But these eggs, in some cases at least, are 

 already impregnated, and somewhat far advanced in developement before 

 I hey Leave the oviduct. I have myself observed, in several individuals 

 which had been some years in spirit, eggs developed as far as the first 

 t,tage figured (47 a), which had never left the ovisac. In this connection 

 it may be observed that, if the sperma be ejected into the water, there 

 is nothing to prevent the spermatozoids from entering the wide aper- 

 tures of the fenestra} (in some species at least), and thus impregnating 

 the eggs in the ovary. In this way may be explained the presence of 

 spermatozoa in the female Chiton Pallasii noted by Middendorf. In 

 some species with very small ovarian openings, this internal impregna 

 tion would be attended with more difficulty. In some species, a large 

 so-called u slime gland" is present, lying under the middle line of the 

 ovisac. Gray states that in some Chitons the egg is enfolded in a thick, 

 vesicular, folded envelope, but I did not notice anything of the kind in 

 those I examined. I observed no micropyle, but having only specimens 

 hardened by long immersion in spirits these observations cannot be 

 deemed conclusive. In all the species specially examined to determine 

 the character of the ova, the eggs were spherical, with a rather tough 

 skin, quite smooth, with no trace of lime in it, and apparently in no 

 way attached to the walls of the ovary when ripe.* 



Developement. — Nothing later than the brief but admirable researches 

 of Loven, now thirty years old, has come to hand. His figures are here 

 reproduced, with a summary of what has been observed, to stimulate 

 further enquiry in those favorably situated. 



The Chitons differ from most Mollusks in that the shell does not appear 

 on the embryo until some time after they are hatched. In this connee- 

 tion, the observations of Krohn on Mwsemidm may be referred to. 



The embryo of Chiton cinerens is oval, with no trace of shelly valves 

 or depressions for them, and is divided into two nearly equal parts by a 

 transverse depression, the margins of which are ciliated. On the middle 

 of the upper part is a tuft of filaments which move slightly. At each 

 end of the depression are two dark points, representing the eyes. 



The young when hatched (Fig. 47 &, c) become more elongated, the 

 front part is finely ciliated, and the tuft occasionally vibrates. The 

 hinder part extends more rapidly and becomes conic. The back is 

 marked by seven furrows ; between these the first rudiments of the shelly 



*Dr. v. Ihering describes the egg of C. squamostts as covered with peculiar thorns, 

 five-sided solid columns, expanding at tin; distal end into a cup whose edge is cut into 

 live points. C. Cgjctaniis and fascicularis had eggs covered with a grooved and irregu- 

 larly furrowed membrane, as described by Gray, but without thorns. The vesicular 

 membrane thus may be considered as a chorion. In the immature stages, the oggs are 

 enclosed in follicles of the tissues of the ovisac. 



