rg BIOLOGICAL LECTURES. 



just sufficient to produce a normal embryo (Pluteus). The 

 methods I employed to determine this limit were as follows : I 

 first followed out the development of single small fragments 

 of protoplasm in a drop of sea- water that was protected from 

 evaporation. As such observations are naturally limited to a 

 small number of cases, and as in a single drop development 

 does not, as a rule, go on for more than two days, 'I based my 

 numerical results upon large cultures kept in larger vessels. 

 From these I ascertained by measurement the ratio of the size 

 of the smallest Plutei to the average size of the Plutei that 

 came from normal eggs of the same culture. By doing this 

 carefully every day and by comparing a great number of cul- 

 tures, the relative size of the smallest Plutei was determined 

 with a sufficient degree of accuracy. Finally I watched the 

 development of small particles in these cultures. These obser- 

 vations, which I carried on during two months last year, and 

 for about the same length of time this summer, gave results 

 which are very definite, as follows: (i) The smallest normal 

 Plutei had about one-half the linear dimensions of the average 

 Pluteus of a normal egg of the same culture. Their volume, 

 therefore, was about one-eighth of that of a normal Pluteus. 

 (2) Smaller fragments developed into a blastula, but then either 

 stopped developing, or reached the gastrula stage much later 

 than normal pieces. In the best specimens of the latter kind 

 some spicules were deposited, but the organism kept its spher- 

 ical form, and did not develop into a normal Pluteus. The 



small embryos that re- 

 mained in the blastula or 

 gastrula stage were just 

 as lively as the normal 

 Plutei, and lived just as 

 long as these. 



Fig. i shows the con- 

 FlG ' dition of an ovum whose 



membrane was made to 



burst before segmentation had taken place. It is divided into 

 twelve cells. The cells a and b of the extra-ovate developed 

 a few hours later into the blastulae a and b of Fig. 2. The 



