Preliminary Note on the Embryology of Crepidula 

 fornicata and of Urosalpinx cinerea. By E. G. Conki.in. 



Crepidula foriiiatPi. 



The time and manner in which Crepidula lays its eggs has been described 

 by Dr. McMurricli.' 



The clea\-age follows tlie type found in Fnsus, Planorbis, Neritina, etc. 

 The first furrow is, with regard to the future embryo, a median longitudinal 

 one and divides the ovuui into exactly equal right and left halves. (The 

 method of determining the relation of the first furrow to Ihe.embryo cannot 

 be explained without several figures). The second furrow is at right angles 

 to the first and transverse to the long axis of the embryo, and it divides the 

 egg into four equal macromeres. Of these, two meet in the centre in a line 

 which Rabl' has called the " cross furrow ; " the other two are acute toward 

 the centre and do not meet each other. By the position of the macromeres 

 with regard to the " cross furrow," the first and second cleavage furrows 

 may easily be distinguished, e.g., if the egg be viewed from the formative 

 pole and so that one of the cleavage furrows is in the line of vision, the 

 macromere to the right of this furrow and farthest from the observer will 

 be acute at its centre if the furrow in the line of vision be the second 

 cleavage furrow ; it will be obtuse, s. «., will meet the opposite macromeres 

 in the " cross furrow " if the furrow in the line of vision be the first cleav- 

 age furrow. Of course the reverse would hold true if the egg were viewed 

 from the vegetative pole. The examination of many hundreds of eggs has 

 shown that the position of the macromeres in relation to the "cross furrow " 

 and to the first and second cleavage planes is a constant one, and that the 

 first and second furrows may always be distinguished in the way mentioned. 

 The macromeres as distinguished from the niicronieres do not agairr divide 

 until late in the course of segmentation, and as they do not change their 

 relative position it becomes very easy to orient all the future furrows and 

 cells with reference to the first two furrows. Tlie polar bodies which are 

 at the centre of the ectodermal area also mark tlie centre of the dorsal sur- 

 face of the embryo, while the blastopore closes and the definitive mouth 

 appears almost directly opposite them. Until after the formation of the 

 blastopore the chief axis is the one drawn from the centre of the ectodermal 

 to the centre of the endodermal area ; with regard to this axis the egg 

 is for some time radially symmetrical; the chief axis of the embryo (the 

 antero-posterior) is at right angles to the chief axis of the egg. At an 

 early period there is a trace of a segmentation cavity, which however is 

 soon obliterated. The formation of micromeres proceeds with wonderful 

 regularity, following the law formulated by Rabl,* new micromeres always 

 .being formed in fours either by being cut off' from the four macromeres or 

 by the division of four micromeres already formed. When twenty-four 

 micromeres have been formed one of the macromeres, which later develop- 

 ments show to he on the posterior side of the ovum and to the left of the 

 median line, divides into a larger ventral and a smaller dorsal moiety. The 

 smaller cell, which, unlike the ectoderm cells, contains yolk, moves to the 

 right until it comes to lie at the posterior end of the median furrow between 

 the two po.sterior macromeres; it very soon divides into a right and a left 

 half, and at a later stage these two cells give rise to two mesoblastic bands 

 which extend forward over the right and left sides of the dorsal surface. 



Save for these mesoblasts the radial symmetry at the stage with 

 thirty -six micromeres is perfect. At this stage twelve of the 



1 A contribution to the Embryology of the Marine Prosobranclis. Studies from Biol, 

 tab., J. H. U., Vol. 3. _ 



2 Carl Rabl, Ueber die Kntwicklung der Tellerschnecke. Morph. Jahrbuch, Bd. 5. 



