BIGELOW: EARLY DEVELOPMENT OF LEPAS. 97 



these abnormalities to the action of chemical impurities and to lack of 

 oxygen. The respiratory movements of the animals are more sluggish 

 when they have been kept several hours in aquaria, and hence the eggs 

 in the mantle chamber may fail to get a sufficient amount of oxygen. 

 It is well known that such abnormal conditions may affect great modi- 

 fications in otherwise regular cleavage. 



Orientation of the Embryo. 



It has already been stated that in the four-cell stage a line drawn 

 through the nuclei of the cells b* and d 8 coincides with the longitudinal 

 (antero-posterior) axis of the future embryo, the cell d 3 being posterior. 

 This relation is shown in the orientation on the plate of Figure 31, from 

 which it also appears that the first cleavage plane is oblique to the same 

 axis. The chief axis of the egg coincides with the dorso-ventral axis of 

 the future embryo, the second polar cell at the animal pole being dor- 

 sal. The spherules of yolk are at the opposite pole of the yolk-bearing 

 cell, thus marking the vegetative pole and the ventral side of the em- 

 bryo. The blastopore later appears on this surface near the posterior 

 end of the egg. 



The anterior end of the embryo lies, as several investigators have 

 noted, at the rounded end of the vitelline membrane. In the four-cell 

 and later stages the long axis of the vitelline membrane and that of the 

 future embryo apparently coincide, but in the two-cell stage the long 

 axis of the future embryo is oblique to that of the vitelline membrane. 

 The long axis of the embryo is brought into coincidence with that of 

 the vitelline membrane when the cells adjust themselves after the com- 

 pletion of the second cleavage (compare Figs. 31 and 32). 



The animal and vegetative poles, which are marked respectively by 

 the second polar cell and the mass of yolk spherules, have a constant 

 relation to the blastomeres and to the planes of cleavage, and I have 

 made use of them as a basis for orientation. Previous investigators of 

 the cleavage of cirripede ova have recognized no definite and constant 

 points of orientation. In 1896 I pointed them out in the cleaving 

 ovum of L. fascicularis ; since then I have found that the polar cell 

 has exactly the same relations to the embryonic cells in all the stages of 

 cleavage in four species of Lepas and in Pollicipes polymerus. 



