68 MR. W. K. BEOOKS ON LUCIFER: 



time before the stage shown in the figure was reached ; but the division of the 

 endoderm cells appears to go on much more slowly than that of the ectoderm cells. 



This phenomenon, the alternation of periods of rest with the periods of active 

 segmentation, does not seem to have received from embryologists the attention which 

 it deserves. A number of observers have pointed out that in many animals, among 

 the Mollusca especially, the distinctness of the spherules becomes more or less com- 

 pletely obscured after each division, and that this state persists until just before the 

 next division, when the spherules swell out and again become conspicuous. The 

 change of form does not seem to be at all general, and in most accounts of segmentation 

 nothing of the kind is recorded. 



I believe that it is a secondary phenomenon, and that the essential thing is the 

 alternation of rest with activity ; and I am confident that careful time records of 

 segmentation will show that this occurs in nearly every case, sometimes with and 

 sometimes without the accompanying change of form. 



I have observed it in Physa, Limncsus, and Planorbis, where segmentation is total 

 and nearly regular ; in the Oyster, where the egg has a rudimentary food-yolk and 

 segmentation is irregular ; in a bony fish with a large food-yolk and a discoidal seg- 

 mentation ; and in Lucifer, Other investigators working under my guidance have 

 observed it in Amllyxtoma and in oligochsetous and polychretous Annelids. These 

 are all the cases in which I have been able to test the matter since my attention has 

 been attracted to the subject ; and as the alternation was found to occur in every case, 

 although the animals are so widely separated and present such diverse modes of 

 segmentation, I feel justified in assuming that the phenomenon is general, and will be 

 found in all eggs which can be properly examined by watching and timing them while 

 segmentation is going on. 



The cause of rhythmical physiological change is an extremely interesting question ; 

 and as the segmenting egg exhibits the phenomenon in the greatest possible simplicity, 

 it would seem to be a peculiarly favourable subject for investigation. 



The phenomena which have been described seem to show that segmentation is not 

 clue to the action of any purely molecular force, like polarity, but is essentially a vital 

 activity, and in a paper on the embryology of the fresh-water Pulmonates (' Studies 

 from the Biological Laboratory of the Johns Hopkins University,' vol. i., part ii.) 

 I have ventured the following explanation. 



During the period of segmentation the protoplasm of the whole egg (of Physa) 

 gradually becomes more and more transparent, on account of the gradual disappear- 

 ance of the granular food-material which it contains, and the rhythmical character 

 of the process of segmentation would seem to admit of a simple explanation on the 

 supposition that the physical properties of the protoplasm offer a resistance which 

 must be overcome before the force which is set free by the assimilation and reduction 

 of the food-material can exert itself to bring about the active changes of segmentation. 

 During a period of rest the process of digestion and assimilation accumulates a store 



