100 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



granules or larger masses. 1 as described by Crampton, which wander out into the 

 cell body. Crampton has observed that these granules give rise to the yolk spher- 

 ules which first appear in the protoplasmic ground substance around the nucleus, 

 leaving the peripheral layer of the egg free from yolk. 



I am of the opinion that the peripheral layer also contains portions of the 

 archoplasm or sphere material; the staining reactions of this layer are like those 

 of the archoplasm; in the disintegration of the sphere flocculcnt masses of archo- 

 plasm pass into this layer; finally, comparison with other forms favors this view. 

 The careful observations of Foot ( 1 S < > ) on the yolk nucleus and polar rings 

 of Allohbophora show that in this animal the polar rings may he traced back 

 step by step to a substance in the vicinity of the nucleus of the very young ovo- 

 cyte, which Foot identifies with archoplasm. In the later stages this substance 

 becomes distributed throughout the cell and forms a more or less irregular peri- 

 pheral layer; finally the substance of this layer aggregates at the two poles of the 

 egg to form the polar rings, as previously described. 



Among gasteropods the sphere material is largely of nuclear origin, containing 

 nuclear sap and dissolved oxj'chromatin, which have escaped from the nucleus 

 during the period of mitosis (Conklin, 1902) ; if the same be true of the ascidians 

 both the peripheral layer of protoplasm (mesoplasm) and the yolk (endoplasm) 

 contain elements which were ultimately derived from the nucleus at the last ovo- 

 gonic division. 



The clear protoplasm (ectoplasm) which is apparent in the egg after matura- 

 tion, and which, in the course of development, passes mainly into the ectoderm is 

 largely contained within the nucleus of the ovocyte. In the first maturation divi- 

 sion an extremely large quantity of nuclear sap, containing an unusual amount of 

 dissolved oxychromatin, escapes into the cell body where it can be recognized as an 

 area of clear protoplasm. This clear protoplasm can be followed through a large 

 part of the development, both in ascidians and in gasteropods. In the latter parti- 

 cularly this clear nuclear plasm is plainly visible in the living egg. It forms a 

 fusiform or columnar area around the first maturation spindle, and after the formation 

 of the polar bodies it flattens out at the surface of the egg, forming first a cone, 

 then a lenticular mass, and finally a cap of clear protoplasm. This cap extends 

 down over the egg to a region a little below the equator, and finally during cleavage 

 it is largely localized in the three quartets of ectomeres. 



In the ascidians the later history of this nuclear plasm is not so easily followed 

 as in the gasteropods, owing to the presence of a peripheral layer of mesoplasm. 

 and to the fact that its movements here are more extensive and complicated. 

 In Cynthia it flows to the lower pole along with the yellow mesoplasm. then it 

 moves with the sperm nucleus to the posterior side of the egg and finally to its 

 center. Here it surrounds the cleavage spindle, and at the close of the fust 

 cleavage moves toward the animal pole so that the larger part of it comes to 

 lie in the upper hemisphere. In subsequent divisions it surrounds all the nuclei 

 though the most of it goes into the ectodermal cells as in the case of gasteropods. 



'These fragments of the yolk nucleus are larger and more easily seen in Molgula than in either 

 " or Cynthia. 



