44 Edmund B. JVilson. 



quantitative, it is still subject to the operation of a regulative factor 

 that lies behind the topographical distribution of the egg-mate- 

 rials. This appears to me one of the most significant results that 

 my experiments have yielded. 



The embryos may in succeeding stages cleave in every detail 

 like whole eggs. Typical 4-cell stages are shown in Figs. 73a, 

 78b, 83, 8-cell stages in Figs. 72, 73b, 84, and the fourth cleav- 

 age, with the formation of the first somatoblast, in Fig. 73c. Fig. 

 73a shows the third cleavage with the formation of the third 

 polar lobe. Many individuals were observed showing the forma- 

 tion of the second polar lobe in normal fashion, though none are 

 figured. 



The larvae arising from fragments of this type differ as mark- 

 edly from those derived from the upper fragments as does the 

 cleavage. Although many of the embryos perish, and of those that 

 live many are abnormal, they frequently possess both the apical 

 organ and a post-trochal region; and occasionally a dwarf larva 

 is produced that is essentially similar, except in size, to an entire 

 trochophore. One of the best of these is shown in Fig. 59, which 

 arose from a lower fragment obtained by oblique section, slightly 

 smaller than half the volume of the egg, and including the whole 

 of the polar area. The typical trefoil stage of this larva is shown 

 in Fig. 57; it has exactly the normal proportions, and segmented 

 normally in later stages. This larva Is somewhat less pointed 

 posteriorly than the normal, but the whole larvae vary consid- 

 erably in this regard. It swam in quite normal fashion. Another 

 larger larva from a lower fragment in shown in Fig. 6^. The 

 total preparation of this larva shows with great clearness a typ- 

 ical apical plate at the upper pole. Out of a very large num- 

 ber of operations I have obtained altogether not more than five 

 or six such perfect larvae, at least half the embryos dying during 

 the cleavage, and a large proportion becoming abnormal during 

 the later development. 



That so large a proportion of the embryos die or develop 

 abnormally is to be expected when we consider the very different 

 mechanical conditions of surface-tension and the like in these small 

 embryos. The fact remains that abnormal larvae may be pro- 



