PRELIMINARY REMARKS- 
The observations recorded in the present Memoir were made chiefly in the spring 
of 1849, while assisting Prof. Agassiz in embryonic investigations upon several other 
types of the animal kingdom. The microscope used was his large Oberhauser. 
A historical sketch of these investigations was read before the American Association 
for the Advancement of Science, at its Cambridge meeting'in August 1849, and printed 
in its Proceedings in 1850.* 
I then contemplated publishing without much delay the whole of my Researches 
upon the American Nemertians and Planarians, together with the embryology of their 
different types. But the time which I could devote subsequently to these investiga- 
tions having become, by necessity, very much limited, and fearing that the chief object 
which I had in view in tracing them, and which was the starting point to new re- 
searches, would lose its actuality, I determined to issue these as a first part. 
The species, the eggs of which constitute the subject of this memoir, is quite abundant 
in the harbors of Boston and Beverly, Mass., under the stones of the beach, where I 
have first observed it in the spring of 1847, and was since described by me under the 
name of Planocera elliptica. I am inclined to think that its range may be found to 
extend further north, and likely also further south. 
All the drawings, except figs. 94 — 103, have been executed by myself from .specimens 
under the microscope ; the latter were made by Mr. A. Sonrel, from two models 
which I furnished him, representing the young Planaria immediately after its escaping 
from the egg. My original drawings have been admirably well reproduced on stone 
by the above named artist. 
C. Girard. 
Cambridge, Mass., September 1850. 
* Proc. Amer. Assoc. Adv. Sc., 2d meeting 1849 (1850), 398, a French version of which subsequently appeared 
in the “ Bulletin de la Societe des Sciences naturelles de Neuchatel (Switzerland), vol. ii. 1850, 300. 
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