Section VI 



CHAPTER 3 



Selected Invertebrates 



RAY L. WATTERSON 



Progressive differentiation involves changes 

 in the constitution of regions of the egg or 

 of cells or groups of cells which amount 

 to either permanent gains or permanent 

 losses. These changes are accompanied by 

 temporal and spatial restrictions of potencies 

 of the several parts of the egg and embryo 

 as development proceeds. 



VISIBLE DIFFERENCES ALONG THE 

 ANIMAL-VEGETAL AXIS OF POLARITY 



The earliest visible differences in the eggs 

 of many aquatic invertebrates are those 

 which occur along one heteropolar axis, the 

 animal-vegetal axis of polarity. Similarly, 

 the earliest invisible differentiation in these 

 eggs, as revealed by experimental analysis, 

 usually occurs along this same axis. This 

 axis stands revealed in a variety of ways. 

 Some eggs are more or less elongated along 

 the animal-vegetal axis. The specific weight 

 of the animal half is frequently less than 

 that of the rest of the egg. The germinal 

 vesicle is usually displaced towards the 

 animal pole. Maturation spindles form nor- 

 mally at the animal pole and the polar 

 bodies mark the animal pole insofar as they 

 remain at their point of origin. In many 

 forms a micropyle exists in the chorion at 

 the animal pole, in others at the vegetal pole. 

 More or less striking accumulations of plasm 

 (pole plasms) may occvir at the animal pole 

 or at the vegetal pole, or at both. Distinct 

 transverse pigment bands may appear per- 

 pendicular to this axis. In some eggs, whether 

 or not a micropyle exists, the sperm enters 

 preferentially in the animal half, in others 

 at the vegetal half; this is a further indi- 

 cation of structural and/or physiological 

 differences at different levels of this axis. 



Visible differences along the animal- 

 vegetal axis arise at different developmental 

 stages in eggs of different animals. In some 

 eggs, for example those of Dentalium, a 



mollusk, such differences are evident before 

 maturation occurs (Wilson, '04a); in these 

 living eggs there is recognizable a white 

 animal pole region, a broad middle trans- 

 verse pigment girdle, and a white vegetal 

 pole region. In other eggs, changes occur 

 during matviration which reveal the axis 

 of polarity more clearly. For example, in 

 Paracentrotus, a sea urchin, the immature 

 egg contains a uniform distribution of red 

 pigment in the ectoplasm, but after matura- 

 tion this pigment is concentrated and re- 

 stricted to a distinct subequatorial transverse 

 band (Boveri, '01) which has proved to be 

 so useful as an indicator of polarity in the 

 hands of Horstadius. Similarly, in the leech, 

 Clepsine, the visibly distinct animal and 

 vegetal pole plasms are first in evidence 

 after maturation; prior to this time the dis- 

 tribution of yolk and plasm provides no 

 visible evidence of polarity (Schleip, '14). 

 In still other eggs, for example those of the 

 tunicates, the polar axis is only weakly in- 

 dicated in early stages, but fertilization re- 

 sults in redistributions of visibly different 

 constituents within the egg imtil their strati- 

 fication clearly reveals the polar organiza- 

 tion. Thus in Styela (Conklin, '05a) the yolk 

 then lies at the animal pole, the yellow ma- 

 terial at the vegetal pole, and in between 

 these two layers there is a zone of clear 

 cytoplasm. 



Cleavage planes can usually be described 

 readily in relation to the axis of polarity 

 which is revealed as described above: cleav- 

 age planes cut throvxgh this axis, run parallel 

 or perpendicvdar to it, micromeres are cut 

 off at the animal pole (annelids, mollusks, 

 ctenophores) or at the vegetal polo (echino- 

 derms, ctenophores), etc. (see Costello, Sec- 

 tion V, Chap. 2). 



Even more important than this, the animal- 

 vegetal axis of polarity is the chief axis of 

 differentiation. As a rule the material situ- 

 ated near the animal pole may be traced to 



315 



