682 PATTERNS AND PROBLEMS OF DEVELOPMENT 



streaming, initiated by fertilization, and can be followed through the cell 

 lineage to particular organ systems. In eggs of some other ascidians cyto- 

 plasmic localizations are not visible, but the course of development sug- 

 gests that pattern is essentially similar to that of Styela (Conklin, 1905a, 

 h, c). The cytoplasmic localizations appear to be associated with fertili- 

 zation; but there is still the question whether fertihzation is actually the 

 determining, or merely an activating, factor, with localization resulting 

 from a dorsiventral pattern already present. According to Conklin, the 

 "mesoplasm," which gives rise to mesoderm, is a peripheral layer in the 

 oocyte; "ectoplasm," most of which becomes ectoderm, is in the oocyte 

 nucleus; and "entoplasm," later largely entodermal, is almost central. 

 The spermatozoon enters near the basal pole, apparently not at a definite 

 point; but its path in the egg leads Conklin to conclude that a symmetry 

 pattern is present before fertihzation. 



On the basis of the character of development after fertilization of 

 pieces of unfertilized ascidian eggs a symmetry pattern was postulated 

 in the unfertilized egg by Dalcq (1935, chap. iii). This was regarded as 

 consisting of two bilaterally symmetrical, crescentic areas in the periph- 

 eral cytoplasm of the equatorial region on opposite sides of the oocyte, 

 each extending around four-fifths or more of the circumference and over- 

 lapping each other laterally. One, the "neurochordoplasm," lies slightly 

 deeper than the other, the "mesochymoplasm" (Fig. 2ig,A,B). A medio- 

 lateral gradient pattern is also postulated in each of these crescents. Al- 

 though, according to a personal communication. Professor Dalcq does 

 not now regard these figures as representative of the true organization 

 of the ascidian egg, they are given here with his permission because they 

 provide, in a measure, a scheme of pattern intermediate between that 

 postulated by Conklin and what Dalcq regards as a more adequate 

 scheme resulting from more extended investigation (Dalcq, 19385). 

 Through the personal kindness of Professor Dalcq, which is here grate- 

 fully acknowledged, a drawing provided by him showing this later scheme 

 is reproduced as Figure 219, C, and the explanation beneath the figure 

 is given in his words. Dalcq regards the cytoplasmic movements associ- 

 ated with fertilization as dependent on this pre-existing pattern, but the 

 origin of whatever pattern is present remains completely obscure. What- 

 ever its origin and nature, the symmetry pattern seems to be a highly 

 effective factor in development. The first cleavage plane is median, and 

 cleavage is bilateral, as far as followed in detail, but earlier cleavage 

 planes do not coincide with the boundaries of the cytoplasmic areas. 



