ORGANIZATION OF THE SEA URCHIN EGG 27 



the adult is represented in the egg by some special chemical make-up- — and this 

 is the idea of preformation expressed in modern terminology — or we are 

 dealing with an egg containing unorganized protoplasm which is gradually 

 changed into the organized structures of the adult. In other words, this is 

 the epigenetic viewpoint. The first concept involves a mosaic, or a pattern 

 of organ-forming substances — special substances which determine each and 

 every organ. The second concept pictures an unorganized protoplasm which 

 needs an organizing factor of some sort to convert it into the various struc- 

 tures of the adult. 



Organization of the sea urchin egg 



Before considering the further implications of these two possibilities, 

 let us examine a special case — that of a sea urchin — in which the egg shows 

 a definite polarity indicated by a band of pigment composed of a large 

 number of small red granules (Fig. 10). One pole of the egg is termed the 

 animal pole; the opposite pole of the egg is termed the vegetal pole; and 

 the line connecting the two is called the animal-vegetal axis. In this egg the 

 essential parts for our analysis are a thin outer cortex, a fluid, internal 

 endoplasm, and a band of red pigment granules below the equator of the 

 egg which we will call the pigment band. The nucleus may be in any position 

 in this cell. 



Figure 10 shows a diagram of the early development of this sea urchin 

 egg. Now, in the unfertilized egg the pigment band marks a specific region 

 of the egg so that by watching this band during development we can see 

 what develops from this region. We can see what structures are derived 

 from the protoplasm that is marked by the pigment band. And this method 

 of tracing structures is indicated in the developmental stages in Figure 10. 

 We begin with a stage where the sea urchin egg is shown with the animal 

 pole, A, upward, the vegetal pole, V , downward, and this relation is main- 

 tained all through the diagrams. The pigment band, which we are going to 

 use as a marker of a specific region in the egg, is just below the equator. If 

 we follow this band through the cleavage stages we see that the first cleavage 

 cuts this band in two. That is, it passes through the animal-vegetal axis. 

 During all the cleavage stages this band keeps its same position relative to 

 the animal and vegetal poles. 



At a later stage, eight cells, we can still see the pigment band, and now 



