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into the embryo to take place as long as the embryo elongates. On 
the other hand I hold that the mass of material to form the embryo 
appears very early and subsequently receives no additions laterally as 
the embryo elongates, and that the elongation takes place by the drawing 
out longitudually of this mass. After Hertwie’s conclusion he deems 
necessary to explain the yolk exposure in the middle of the divided 
nerve chord. I have tried to show above that we are not forced to 
this interpretation and I should attempt to explain all such exposures 
as due to a division in the posterior extension of em- 
bryonic tissue and believes that these divided portions subse- 
quently, i.e. of the dorsal lip of the blastopore, unite around the ob- 
stacle in a single structure again medium in position. 
Perhaps I have stated my conclusion too positively. Any one 
working at such problems will realize and appreciate the difficulty of 
correct interpretation of such evasive and complicated phenomena. I 
should wish therefore to offer the explanation attempted above as an 
alternative view that may help as a working hypothesis and give a 
stimulus to further inquiry along these lines. 
The Marine Laboratory, Woods Holl, Mass. 
July 28, 1893. 
Die Herren Mitarbeiter werden dringend gebeten, ihre Wünsche 
bez. der Anzahl der ihnen zu liefernden Sonderabdrücke auf das 
Manuscript zu schreiben. Die Verlagshandlung wird alsdann die 
Abdrücke in der von den Herren Verfassern gewünschten Anzahl 
unentgeltlich liefern. 
Erfolgt keine andere Bestellung, so werden fünfzig Abdrücke 
geliefert. 
WEE” Dieser Nummer liegen Titel und Inhaltsverzeichnis des 
VIII. Jahrganges bei. — No. 1/2 von Jahrgang IX erscheint Mitte 
October d. J. 
Frommannsche Buchdruckerei (Hermann Pohle) in Jena. 
