36 BULLETIN : MUSEUM OF CO^MPARATIVE ZOOLOGY. 



to the view until the recent iinpoi'tant work of R. von Erlanger 

 ('91) upon Pahidina, in whicli he demonstrates that the mesoderm 

 originates by a pair of ventral out-pocketings, and that there is 

 a close connection, though not contiiinity, of the paired fundaments of 

 the pericardium with these out-pocketings. As yet his results are iso- 

 lated, and the hope that a similar origin of the mesoderm miglit be found 

 in other MoUusca is unfulfilled, at least so far as other investigators 

 are concerned. 



The generally vague and often contradictory results obtained by vari- 

 ous investigators of molluscan embryology concerning the origin of the 

 mesoderm have made very apparent the need of careful and detailed 

 work along the lines laid down by Rabl ('79), Blochmann ('81), Wilson 

 ('92), Couklin ('92), and Heymons ('93). It is only in the light of 

 such work as this that a classification of sweeping import, like that of 

 the Hertwigs' " Coelomtheorie," can find its final justification, if it 

 has one. 



It was with a view of adding something to our knowledge of the 

 details of this subject tliat the work in hand was undertaken. The 

 pursuit of this has led me unavoidably into the study of the cleavage, 

 and to a certain extent into the field of cell lineage. 



Limax seemed for many reasons to be a desirable object for my inves- 

 tigation. The adults are readily procurable, and an abundant supply of 

 eggs whose age is approximately known can be o\)tained from animals 

 kept in confinement. The absence of a large amount of nutritive yolk 

 leaves the eggs sutficiently translucent for examination in toto, and 

 makes section-cutting feasible, though the smallness of the eggs renders 

 their orientation in certain stages diflicult. 



My work was begun in the fall of 1892, at the suggestion of Dr. E. L. 

 Mark, to whose kindly interest and guidance I owe very much. Agrio- 

 limax agrestis L. was the species chosen for the work, as the adults are 

 abundant at that time of the year about piles of rubbish and stone 

 heaps, — in fact, wherever decaying vegetable matter and moisture afford 

 food and a suitable retreat. After the last of November, a supply of 

 adults can generally be secured in greenhouses. 



The eggs of other Limaces can also be collected in the same localities, 

 and as those of Agi-ioliraax agrestis are not readily distinguished from 

 some of them, recourse was had to eggs of known parentage only. 



