PRESIDENT S ADDEESS. 



67 



and this tendency of the yolk- laden cells to cleave first is further 

 evident in JSeritina and in members of Invertebrate classes other than 

 the MoUusca. As Kofoid points out, "a paradox is thns presented. 

 Yolk appears to delay cleavage in the cells of the frog's egg, to 

 hasten it in the cells of the snail's egg." The discovery by Ishikawa ' 

 that among the Daphnida^ the summer egg may be little yolk-laden 

 and holoblastic, and the winter egg rich in yolk and meroblastic, like 

 that of the so-called "dorsal" disposition of the endodermal blasto- 

 meres in some Tunicates ^ ; and the announcement ^ that in Limulus, 

 in which yolk-division would appear to be normally meroblastic, the 

 segmentation of eggs of a batch may be either meroblastic or holo- 

 blastic, in accordance with comparatively trivial changes in position 

 and environment, teem with interest in this association ; and we are 

 led to inquire into the part played by gravitation and specific 

 gravity. History once more repeats itself ; and we Biologists, having 

 to deal with probabilities, have to be content with tentative conclusions 

 — but we are none the further from the truth for all that ! 



Dr. Kofoid' s monograph was immediately followed in time of 

 writing by a short paper by H. E. Crampton, of New York, in 

 which it is shown * that whereas in the dextral Limncea columella 

 cleavage is dextral, in the sinistral Plujsa heterostropha it is reversed 

 and sinistral. 



Dismissing this branch of our subject, with the remark that in 

 respect to it our Japanese friends are not one whit behind,* let me 

 now say a few words concerning one or two special advances which 

 we in part owe to the older, and withal the ever-reliable, methods 

 of comparative anatomy. 



The genus Siphonaria, from the shell of which some of us whose 

 occupation takes us into the examination room have learned much 

 that is instructive and undreamed of by the conchologist, is once more 

 to the front. Ivohler, in an admirable and beautifully illustrated 

 monograph,® has pointed out that its gill, in structure allied to that 

 of the Pleurobranchidte, is in position akin to that of the Bullas. 

 Arriving at the conclusion that this is a true ctenidium, and that the 

 points of structural agreement between the gills of Siphonaria and 

 the Tectibranchiata are indicative of close genetic relationship, the 

 author proceeded to carefully investigate their innervation. Support 

 for his conclusion was obtained, and, finding that in respect to the 

 general organization of the nervous and reproductive systems there 

 were further points of structural community between ITmhraculum, the 

 BuUidae, and Siphonaria, he closes a case for the retention of the latter 

 much debated genus among the Opisthobranchiata. But no sooner is 



' Ishikawa. See paper by "Watase in Journal of Morph., vol. iv, p. 260. 



* Samassa, Archv. Mikr. Anat., Bel. xliv, p. 1. Vf., however, Castle, Ann. New 

 York Acad. Sci., vol. viii, p. 167. 



^ Patten, Zool. Auzeiger, Bd. xvii, p. 72. 



* Proc. Amer. Acad. Arts and Sci., vol. xxx, p. 200. 



* Cf. Mitsukuri, Anat. Anzcioer, Ikl. xi, p. 406. 

 ® Zool. Jahrb. Anat. Abth., Bd. vii, p. 1. 



