No. I.] THE EMBRYOLOGY OF CREPIDULA. 45 



not lie in either of these planes, but is oblique to both of 

 them. Rabl ('79) calls this in Planorbis the "cross" or trans- 

 verse furrow (" Querfurche "), indicating thereby that it lies 

 transverse to the long axis of the embryo. Blochmann ('81) 

 also mentions this furrow as being present in the o.^^ of Neri- 

 tina, and describes the method of its origin. He calls atten- 

 tion to the fact that it lies in the transverse plane of the 

 embryo ; and he considers that it is caused by the difference 

 in the time of division of the two cells. But that this is not 

 generally the case, is shown by the fact that it is present in 

 many eggs in which the division of the first two blastomeres 

 occurs simultaneously. Rauber ('82) has described at some 

 length a similar furrow, which is found in the frog's ovum, as 

 well as in Petromyzon and Gobius. He calls it the breaking 

 line (" Brechungslinie "), and says that it may be formed in two 

 ways : (i) the second furrow really consists of two furrows, 

 one of which divides one of the first two blastomeres, the other 

 the other one ; these two furrows may or may not meet in the 

 centre ; in the latter case the breaking line is formed ; (2) if a 

 breaking line is not formed at first, it may appear later by the 

 shifting of the blastomeres. While Rauber considers that the 

 position of the breaking line has an influence on the subse- 

 quent cleavage, he regards its position relative to the other 

 furrows or to the embryonic axes as purely a matter of chance. 

 As he points out, it is particularly well marked in the four-cell 

 stage of many ova ; at this stage there are often two " cross 

 furrows " on opposite sides of the egg ; these are at right 

 angles to each other, so that each of the four cells is acute at 

 one pole and truncated at the other. O. Hertwig ('80) has 

 also called attention to this furrow in the egg of Sagitta. He 

 says of it : " An dem animalen Pole des Eies, welcher gerade 

 abgebildet ist, stossen nicht alle vier Zellen, wie es bei regel- 

 massiger Furchung der Fall sein soUte, in einem Punkte zusam- 

 men, sondern nur zwei derselben beriihren sich mit verbreiterten 

 Enden und bedingen eine kurze gerade Furche, welcher wir 

 ihrer Lage nach als Polarfurche benennen wollen ; die 

 beiden anderen Zellen, welche von der gegenseitigen Beriih- 

 rung ausgeschlossen sind, enden zugespitzt an den beiden 



