ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 97 



clear protoplasm at each pole of the egg with a broad pigment band around the 

 equatorial region. The clear polar areas, the lower of which forms a prominent 

 lobe, Wilson regards as comparable with the "polar rings" of leeches and oligo- 

 chaetes. 1 In the course of development the upper white area is allotted to the three 

 quartets of ectomeres; the middle pigmented zone is mainly allotted to the four 

 basal entomeres, while the lower zone passes mainly into the first somatoblast (2d), 

 and possibly also into the second somatoblast (4d) and the left posterior micromere 

 (3d). This work is the most complete and important which has yet been done on 

 the subject of cytoplasmic localization and it firmly establishes the fact that differ- 

 ent substances and areas of the unsegmented egg are causally related to different 

 organs and parts of the larva. 



It is doubtful whether any other case of cytoplasmic localization hitherto 

 reported is more remarkable than that which has been described in the preceding 

 pages for the ascidian egg. The most striking features of this localization are the 

 great differences in the substances localized, the manner in which this localization 

 is accomplished and its bilateral character. 



(1) The first of these features is the result of the different pigments which 

 are assoeiated with the different kinds of protoplasm, and which mark out as on a 

 map the various germinal areas of the egg. In Cynthia the pigment in the periph- 

 eral layer of protoplasm is yellow, the yolk is a blueish gray, while the protoplasm 

 which escapes from the germinal vesicle is colorless. Not the pigment but the pro- 

 toplasm with which it is associated is of differential value, for the pigment may 

 differ most remarkably in different genera of ascidians. but the organs which arise 

 from similar areas are in all eases similar. What has been said of the pigment may 

 also be said of the yolk; this inert substance is not in itself of differential value, 

 but it lies in a definite region of the egg and probably in a particular kind of proto- 

 plasm, which it marks out as the yellow pigment does the peripheral layer. 



Of these three kinds of protoplasm the yellow (mesoplasm) goes almost entirely 

 into the muscle and mesenchyme cells, though a small portion of it may be found 

 around the nuclei of other cells, the clear protoplasm (ectoplasm) is chiefly distribu- 

 ted to the ectoderm and the gray yolk-laden protoplasm (endoplasm) 2 to the endo- 

 derm, though here also some of these substances are distributed to all the cells. It 

 is not to be supposed that these three kinds of protoplasm are the only ones present 

 in the Q^iX. rather it is probable that others are present which are not visibly distin- 

 guishable. In fact, soon after the cleavage begins, it is noticeable that the proto- 

 plasm in the dorsal part of the crescent is a fainter yellow than that in the ventral 

 part, while from the time of the fertilization onward the middle of the crescent is 

 marked by a small area of clear protoplasm {v. p. 21) ; the deeply pigmented 



Several years ago I suggested (Conklin 1897, p. 39) that the yolk lobe (" polar lobe," Wilson 

 was comparable to the polar rings of leeches. 



- It should lie observed that these names are given with reference to the part which these 

 different portions of the ooplasm play in the development of the animal; the peripheral layer of the 

 ovocyte, which would be called ectophtzm if the ovocyte alone were under consideration, is mesoplasm 

 when regarded from the standpoint of its fate in development. 



13 JOURN. A. X. S. PHILA., VOL. XIII. 



