46 CONK LIN. [Vol. XIII. 



Enden der Polarfurche. Ganz dieselben Verhaltnisse wie- 

 derholen sich am vegetativen Pole ; nur treffen sich hier die 

 beiden Zellen, welche den animalen Pol nicht erreichten, mit 

 verbreiterten Enden. Sie bilden eine vegetative Polarfurche, 

 welche die animale, wenn wir beide auf dieselbe Ebene proji- 

 ciren, unter rechtem Winkel kreuzt, wie man beim Wechseln der 

 Einstellung an dem durchsichtigen Object leicht feststellen 

 kann. . . . Eine ahnliche Anordnung der vier ersten Furchungs- 

 zellen wie bei Sagitta hat soeben auch Rabl an den Eiern von 

 Planorbis genau beschrieben, er nennt die Polarfurche Quer- 

 furche und bemerkt hierzu, dass sie einen wichtigen Anhalts- 

 punkt fur die Orientirung des Keimes abgiebt." 



In all holoblastic eggs which are laden with yolk the polar 

 furrow at the vegetal pole is much longer than the one at the 

 animal pole, — in fact, the latter maybe absent altogether, as is 

 the case with Crepidula. In using the expression " polar furrow" 

 in connection with this animal, it must be understood to refer 

 only to that structure which Hertwig calls " vegetative Polar- 

 furche." As just remarked, the name " Querfurche " seems 

 to have been given in the belief that this furrow is always 

 transverse to the antero-posterior axis of the embryo, as it is 

 in Planorbis and Neritina, and as I have found is the case in 

 Urosalpinx and Tritia. If one may judge from the figures 

 alone this seems to be its position in Nassa and Fusus, as 

 described by Bobretzky ('77), and in Vermetus, studied by 

 Salensky ('87). In all forms, however, in which the first cleavage 

 coincides with the antero-posterior axis, or is at right angles to 

 it, the furrow in question could not be transverse to that axis, 

 but would necessarily be oblique to it ; this is its position in 

 Nereis, Umbrella, and Crepidula. In such cases the name 

 " cross furrow " is evidently a misnomer. The furrow bears no 

 constant relation to the axes of the embryo, being at one time 

 transverse and at another oblique to the longitudinal axis ; 

 and it is just as illogical to name this furrow from its relation 

 to the axes of the embryo as it would be to name the first 

 cleavage from such a relation, which in some animals coincides 

 with the antero-posterior axis, in others is at right angles to it, 

 and in still others is oblique to it. 



