OUR EMBRYONIC DEVELOPMENT. 61 



sexual means, by fission, gemmation, or spores ; they 

 have no real ova and no sperm. The metazoa, on 

 the contrary, are divided into male and female sexes, 

 and generally propagate sexually, by means of true 

 ova, which are fertilised by the male sperm. 



III. — Hence, further, true germinal layers, and the 

 tissues which are formed from them, are found only in 

 the metazoa ; they are entirely wanting in the protozoa. 



IV. — In all the metazoa only two primary layers 

 appear at first, and these have always the same 

 essential significance ; from the outer layer the 

 external skin and the nervous s} T stem are developed ; 

 from the inner layer are formed the alimentary canal 

 and all the other organs. 



Y. — I called the germ, which always arises first 

 from the impregnated ovum, and which consists of 

 these two primary layers, the "gut-larva" or the 

 gastrula ; its cup-shaped body with the two layers 

 encloses originally a simple digestive cavity, the 

 primitive gut (the progaster or archenteron) , and its 

 simple opening is the primitive mouth (the prostoma 

 or blastopores). These are the earliest organs of the 

 multicellular body, and the two cell-layers of its 

 enclosing wall, which are simple epithelia, are its 

 earliest tissues ; all the other organs and tissues are a 

 later and secondary growth from these. 



VI. — From this similarity, or homology, of the 

 gastrula in all classes of compound animals I drew 

 the conclusion, in virtue of the biogenetic law (p. 82), 

 that all the metazoa come originally from one simple 

 ancestral form, the gastnea, and that this ancient 

 (Laurentian), long-extinct form had the structure 

 and composition of the actual gastrula, in which it is 

 preserved by heredity. 



