256 WATASE. [Vol. IV. 



ent stages of growth and give rise to definite axes of the larval 

 or of the adult organism. If these facts be more firmly estab- 

 lished by the further investigation of the subject, we may say 

 with van Beneden : ^ " L'ancienne theorie de revolution ne 

 serait pas aussi denuee de tout fondement qu'on le croit 

 aujourd'hui." 



III. 



It has been pointed out that the cytological study of the 

 animal ovum is different from the study of germ-layers. It 

 has also been pointed out that the morphology of a bilateral 

 organism, at least in those well-established cases, has its begin- 

 ning in the morphology of its bilateral ovum. 



The next important point to be borne in mind, it appears to 

 me, in the study of the cleavage of the ovum, is the phenomena 

 of tissue-differentiation, which become more and more manifest 

 with the growth of the ovum and the progress of its cleavage. 

 Now what is meant by the differentiation of tissues .-* What is 

 the relation of the tissues thus differentiated to the original 

 ovum from which they sprung .'* How does one tissue-cell 

 differ from another, which belongs to another tissue .? 



Strictly speaking, the differentiation of tissues in the devel- 

 oping organism means the formation of two or more different 

 kinds of protoplasm out of one. For the sake of clearness we 

 may begin our inquiry by asking, How does one Qgg which is a 

 single nucleated cell, and which gives rise to one animal, differ 

 from another which gives rise to an entirely different organism > 

 Both are simple nucleated cells, and as such they are morpho- 

 logically alike, but one egg will develop into one organism and 

 the other into another after a repeated division and subdivision, 

 exactly under the same circumstances, and often in the same 

 space of time. The difference in result cannot, therefore, be 

 attributed to difference of conditions under which they develop, 

 but to something inherent in the ova themselves. In other 

 words, the egg-cell of a jelly-fish must have had from the begin- 

 ning the potentiality of becoming a jelly-fish and nothing else; 

 and similarly, the starfish ovum must have been a potential star- 

 fish from the beginning. To imagine, therefore, that all proto- 



^ Van Beneden : Recherches sur la maturatio7t de Vauf, la fecondation et la divi- 

 sion celhdaire. Archives de Biologic, Tome IV, 1883. 



