470 



cleavages of four macromeres, whereas in Lepas there are not 

 three quartets of cells but three cells formeel in the same order 

 of cleavage. In the annelids and mollusks the first segregation 

 of ectoblast from eutoblast is represented by the upper four cells 

 (first quartet of micromeres) of the eight-cell stage, formed by 

 the third cleavage, whereas in Lepas the first segregated ectoblast 

 is one of the two cells formed by the first cleavage. Stated in 

 other terms, in annelids and mollusks, unlike Lepas, the first 

 and second cleavages are not directly concerned with the segre- 

 gation of ectoblast from entoblast, but they divide the egg into 

 a quartet of macromeres, each containing entoblast, from which 

 in succession three quartets of ectoblastic micromeres are sepa- 

 rated. In Lepas the segregation of ectoblast begins, as it were 

 precociously, without the previous division of the entoblast into 

 a quartet of cells. As a result of this there is in Lepas one 

 entoblastic macromere instead of four, as in annelids and mollusks, 

 and single micromeres appear to represent quartets. So far as 

 the order of cleavage involved in the segregation of the primary 

 germ-layers is concerned, the first micromere (a h") of Lepas ap- 

 parently corresponds to the first quartet of ectoblastic micromeres 

 seen in the eight-cell stage of such eggs as have four macromeres 

 resulting from the quartet-forming (first and second) cleavages. 

 The micromeres of Lepas are, then, according to this view, to 

 be regarded as equivalent to quartets of micromeres, while the 

 single yolk-macromere equals a quartet of macromeres." Bigelow 

 neigt also zu der AulFassuug hin, dass die beiden ersten Fur- 

 chungen der Anneliden und MoUusken bei Cirripedien wegfallen 

 und dass die 1. Furchuug der letzteren der 3. Furchung der 

 ersteren entspricht. Dagegen betonen Mark und Castle in ihrer 

 IS^achschrift zu Bigelow's Aufsatz, dass sie dieser Auffassung 

 nicht beistimmen und glauben in der Eifurchung von Lepas „un- 

 mistakable evidences of quadrant symmetry" zu entdecken. Mir 

 sclieint, eine Entscheidung zwischen beiden Auff'assungen kann 

 die Betrachtung der Richtung der 1. Teilung bei den Cirripedien 

 bringeu. Entspricht hier die 1. Teilung der dritten der Anneliden 



