BLASTOPORE AND ANUS IN PALUDINA VIVIPARA. 379 



from the luxuriant old Botanick Garden of the University 

 and for the use of his adjacent laboratory. 



Paludina vivipara is so common a Gastropod and so con- 

 venient for embryological study that there can be little diffi- 

 culty in confuting or confirming by actual study of the 

 object in question statements made relating to its develop- 

 ment. It would, however, be exceedingly difficult to obtain 

 actual sections of its earlier phases ; this is the less to be 

 regretted since the young embryos are far more transparent 

 than are those of most Gastropoda. The notion that only 

 those embryos which admit of the section-cutting treatment 

 can furnish really valuable evidence in embryological ques- 

 tions is a prejudice which needs to be checked at the pre- 

 sent day. The embryologist who has not practised himself 

 in the arts of hardening, embedding, slicing, and clarifying is 

 prone to choose for his studies a certain class of embryos, 

 namely, those which are transparent. Generally it may be 

 said these are the developmental histories in which a small 

 or unimportant amount of food-material (Nahrungsdotter) is 

 present, and are consequently liable to certain common cha- 

 racteristics. On the other hand, the section-cutting embry- 

 ologist is liable to occupy himself exclusively with the largest 

 eggs which he can procure belonging to the particular animal 

 tribe he wishes to investigate. These large eggs are as a 

 rule eggs with a relatively large amount of food material, 

 and present in their development characteristics dependent 

 on this relatively large amount. Consequently, exclusive 

 pursuit of the one method of study is as likely as the other 

 to give results which are arbitrarily though unconsciously 

 " selected.'' 



The embryologist has to be on his guard lest his favourite 

 method of manipulation distort his view of the facts in the 

 one or the other direction. 



Dr. Franz Leydig in one of his classical monographs pub- 

 lished as long ago as 1850 (' Zeitschrift fur wiss. Zoologie,' 

 vol. 2) gives an account of the anatomy and development 

 of Paludina vivipara. Nothing has been published in the 

 past twenty- six years relative to the development of that very 

 common Gastropod except my own note in this Journal in 

 April, 1875. Leydig's description and figures of the earlier 

 stages of the development are correct so far as relates to the 

 actual appearances ; but the use of higher powers of the 

 microscope than those which were available so long since 

 renders it easy to make out at the present day many details 

 as to the arrangement of the cell-elements of the embryo 



VOL. XVI. NEW SER. B B 



