2 I iS EDWIN G. CONKLIN. 



each of which gives rise to some specific portion of the larva and 

 is here present in its definitive position and proportions, viz., the 

 clear protoplasm which gives rise to the ectoderm, the gray sub- 

 stance which produces endoderm, the deep yellow substance 

 which develops into the muscle cells, the light yellow which goes 

 into the mesenchyme, the clear protoplasm at the middle of the 

 yellow crescent which becomes caudal mesenchyme and the light 

 gray substance of the gray crescent which gives rise to chorda 

 and neural plate. Inasmuch as it is difficult to refer to these 

 different substances of the egg by the purely descriptive terms 

 which have been employed thus far, I propose to designate them 

 by names suggestive of the parts to which they ultimately give 

 rise, viz., ectoplasm, endoplasm, myoplasm, chymoplasm, caudal 

 chymoplasm and chorda-neuroplasm. l Of all of these substances 

 the mesoplasm (myoplasm and chymoplasm) alone takes its defin- 

 itive position before the first cleavage ; the other substances reach 

 their final positions only at the close of this cleavage. But 

 although the localization is not complete in the unsegmented egg 

 the ectoplasm and endoplasm are nevertheless clearly differenti- 

 ated before cleavage begins ; I am unable to say whether the 

 chorda-neuroplasm is also differentiated at this stage. 



CLEAVAGE AND GERMINAL LOCALIZATION. 



In the early stages of development it is apparent that the cleav- 

 age planes do not closely follow the lines of separation between 

 the different substances of the egg. The yellow crescent is 

 bisected at the first cleavage ; the second cleavage passes anterior 

 to it ; the third cleavage plane lies some distance above (ventral 

 to) the upper border of the crescent ; the fourth cleavage bisects 

 the halves of the crescent on each side of the median plane. No 

 one of these first four cleavage planes follows any one of the 

 boundaries of the crescent ; the same is also true of all the other 

 ooplasmic substances. Although the localization of these sub- 

 stances is precise and definite, the localization pattern does not 

 correspond to the early cleavage pattern. In the later cleavages 

 some of the division walls do closely correspond with the planes 



1 The substances of the chorda and neural plate are not clearly distinguishable from 

 each other before the S-cell or i6-cell stage. 



