•the results of EMBRYOLOCxY. 583 



entiation ; and the tendency of recent investigations appears 

 to render it very questionable whether the nucleus of the 

 ovum ever really disappears, whatever may be the modiiica- 

 tions undergone by the germinal vesicle and its contents. I 

 shall, therefore, assume provisionally, that the primary form 

 of every animal is a nucleated protoplasmic body, cytode, or 

 cell^ in the most general acceptation of the latter term. 



Whether the primary cytode possesses a nucleus or not, 

 the important fact remains that, in its earliest condition, 

 every invertebrated animal, if it were competent to lead an 

 independent existence, would be classed among the Protozoa. 



The first change which takes place in the development of 

 the embryo from the primitive cytode, or impregnated ovum, 

 in all the Metazoa^ is its division ; and the simplest form of 

 division results in the formation of a spheroidal or discoidal 

 mass of equal, or subequal, derivative cytodes, the blasto- 

 meres. Next, the morula, thus formed, generally acquires a 

 central cavity, the blastocoele, and becomes a hollow vesicle, 

 the hlastosphere, the wall of which, composed of a single layer 

 of blastomeres, is the blastoderm. 



The blastomeres of the blastoderm next undergo differen- 

 tiation into two kinds, distinguished by their internal activi- 

 ties, if not by their outward form. Of these the one set con- 

 stitute the epihlast, the others the hypoblast. The further 

 changes of the embryo are the consequences of the tendencies 

 toward further modification resident in the epiblastic and hy- 

 poblastic blastomeres respectively. Each of these is, as it 

 were, a germ, whence certain parts of the adult organism will 

 be evolved. 



Every series of the Invertebrata has now yielded a num- 

 ber of examples of the further modification of the blastosphere 

 by the process of invagination, or emboly, the result of which 

 is that the hypoblast becomes more or less completely inclosed 

 within the epiblast. The invagination is accompanied by the 

 diminution, or even abolition, of the blastocoele, and the for- 

 mation of a cavity inclosed within the hypoblast, which is the 

 archenteron , or primitive alimentary cavity. The opening 

 left by the approximated edges of the epiblast, when the pro- 

 cess of invagination is completed, and by which the archente- 

 ron communicates with the exterior, is the blastopore. In 

 this state the embryo is a gastrula. 



It very commonly happens that the process of develop- 

 ment is modified by an inequality in the size of the blasto- 

 meres ; which inequality may be manifest from the bisection 



