14 bulletin: museum of COMrAEAXIVE ZOOLOGY. 



for other forms thau Callidiua russeola, whatever may be the conditions 

 in that species. 



3, Orientation of the Developing Embryo. 



The first cleavage plane is transverse to the long axis of the egg, and 

 divides it into two unequal parts (Figure 6). The plane passes through 

 the place where the polar cell was formed ; the smaller cell includes, 

 therefore, that end of the egg nearest to which the polar cell is located- 

 As previously stated, this is also the more pointed end of the egg, when 

 any difference in the two ends is distinguishable. 



The second cleavage is approximately at right angles to the first, and 

 nearly in the long axis of the egg. It also passes through the region 

 •where the polar cell was formed. 



As previously stated, the oval or ellipsoidal form of the "egg is re- 

 tained throughout the early development. This form is independent ot 

 the precise arrangement of the material of which the egg is composed. It 

 is as if the egg substance were enclosed in a rigid mould of oval or ellip- 

 soidal form. AVithin this mould the (fluid ?) contents may shift their 

 position widely, without influencing the form of the mould. Thus, at the 

 first cleavage, the material of the smaller blastoraere occupies all of one 

 end of the egg. In the ten-cell stage (Plate 3, Figs. 20-2.5) the same 

 form is still preserved as if in a rigid cast, bat the material which 

 previously formed the smaller of the first two blastomeres has shifted 

 from the end to one side of the egg. It therefore is necessaiy to have 

 sorr^ term by which to designate the two ends of this constant form, as 

 dist>!iguished from the shifting blastomeres themselves. I shall hence- 

 forth speak of that end of the egg at which lies the smaller cell in the 

 tr 1-cell stage as the micromere end of the egg, while the opposite region, 

 where the larger blastomere lies, will be called the macromere end. 

 These terms refer to the form of the egg, without regard to the shifting 

 contents. 



The orientation which I shall adopt for the egg itself is similar to 

 that used by Wilson, Heymons, Conklin, Lillie, Kofoid, and other re- 

 cent workers on cell lineage. The region where the polar cell is formed, 

 and which afterward lies opposite the blastopore, will be called the ani- 

 mal pole ; it marks the dorsal surfoce. The opposite point is the vege- 

 tative pole, marking the ventral surfoce, — the position of the future blas- 

 topore. Dorsad signifies always toward the animal pole, or place where 

 the polar cell was formed; ventrad, in the opposite direction, toward 



