660 INVEETEBEATA chap. 



by the co-operation of the substances, instead of one. If, for instance, 

 the 2-cell stage of a developing frog's egg be compressed between 

 two glass slides so that it cannot rotate as a whole, and the prepara- 

 tion inverted, a double-headed tadpole will result. Here nothing has 

 been removed, nor have the connections of the blastomeres with each 

 other been destroyed, but the spatial relations of the organ-forming 

 substances have been altered under the influence of gravity, and as a 

 result the tadpole has two heads instead of one. Of a quite similar 

 nature are the results obtained by Spemann (1901, 1903) on the 

 gastrula of the newt, when he employed constriction by a string, and 

 a two-headed monster resulted. So much we can determine empiric- 

 ally, but the how and why are still to seek. 



The reader will see that the door is opened to a host of interesting 

 questions, and that indeed experimental embryology passes into 

 cytology, for the whole question of the relation of the nucleus to the 

 cytoplasm is raised. To take one example of such questions, we may 

 ask, if the nuclei of the segmenting egg are indifferent material, and 

 the distribution of cytoplasmic organ-forming substances is fixed at 

 the moment of fertilization, and if these substances, as seems certain, 

 are of purely maternal origin, at what point in development does 

 the male influence assert itself? For this point must coincide witb 

 the point of renewed nuclear activity. A series of carefully chosen 

 hybridization experiments should throw light on this problem. 



Again, we have seen reason to ascribe the formation of buds to 

 the reassumption of an active role on the part of the nuclei of adult 

 tissue, and the renewed formation of organ-forming substances. We 

 have ascribed the different course of the development of buds from 

 that obtaining in embryonic development to a different distribution 

 of these substances. Can we detect microscopically any difference 

 between these active nuclei and the normal inactive nuclei ? Here 

 again is a fruitful field for research. 



Let us in the meantime, adopting the recapitulatory view, 

 provisionally sketch the developmental history of invertebrates so far 

 as our present knowledge extends. 



In the larval history of the least modified Sponges, Coelenterata, 

 and Echinodermata — groups which, in the differentiation of their adult 

 organs, are the lowliest amongst Invertebrates — the first distinct 

 larval .phase is the blastula, which is a hoUow sphere or ellipsoid, 

 whose walls are constituted by a single layer of flagellated cells. 

 Since this larval phase is represented in a form more or less obscured 

 by secondary changes in the embryonic life-history of all the other 

 groups, we may take it as representing in rudest outline the form of 

 the common ancestor of all Metazoa. It resembles in many respects 

 certain existing colonial Protista, such as Volvox, sometimes claimed 

 by zoologists and sometimes by botanists. 



Such a simple, free-swimming stock must have swarmed in the 

 shallow, warm seas of the primitive globe, when there were no higher 

 forms to compete with it. At first every cell fed itself like every 



