No. I.] THE EMBRYOLOGY OF CREPIDULA. 163 



remaining entomeres he calls the " Secondare Darmzellen."' 

 The intestine is said to be formed from the small cells C" and 

 D" (^ 5C and 5D). In the history of the entomeres this is 

 really the only point of difference between Umbrella and 

 Crepidula which cannot be satisfactorily explained. 



Neither Wilson nor Lillie have observed the division of the 

 entomeres after the formation of the fourth quartette. Mead 

 ('94), however, has observed the formation of the fifth quartette 

 in Amphitrite, for he says, p. 467 : "a4, b4, c4 (=. 4 A, 4B, 4C) 

 and A, B, C, D form the entoderm, the latter cells each 

 dividing once before the invagination." 



VIII. Axial Relations of Egg and Embryo. 

 I. The Primary Cleavages. 



In recent years much attention has been paid to the relation 

 of cleavage planes to the future axes of the developing animal. 

 Interest was first awakened in this subject by several observa- 

 tions which tended to show that the first cleavage plane always 

 coincides with the future median plane. Such a 1 elation was 

 found by Agassiz and Whitman ('84) in certain pelagic fish 

 eggs, by Roux ('85) and Pfliiger ('85) in the frog, by Van 

 Beneden and Julin ('84) in Clavellina, by Watase ('90) in Loligo ; 

 and the belief was expressed by some authors that the first 

 cleavage plane would be found to coincide with the median 

 plane in all animals with bilateral symmetry. However, fur- 

 ther work on this subject has not justified this opinion. 



Hatschek ('80) found that in the case of Teredo the first 

 cleavage was transverse to the median plane, and later the same 

 relation was found by Wilson ('90) in Nereis, Conklin ('92) in 

 Crepidula, Heymons ('93) in Umbrella. 



In other animals it appeared that the first cleavage lay 

 between the median and transverse planes. Whitman ('78) 

 found in Clepsine that one of the first four macromeres is 

 anterior, another posterior, and that both the first and second 

 cleavage planes cross the median plane of the embryo obliquely. 

 Rabl ('79) established such a relation in Planorbis. Blochmann 



