238 ERNST HAECKEL. 



the Zoophyta does the formation of both kinds of sexual 

 cells from the epithelium remain confined to certain parts of 

 the gastro-canal system ; and even in many worms there are 

 still no independent persistent sexual organs present in a 

 morphological sense. In many worms (Bryozoa, Annelida, 

 &c.) individual ccfilom-cells, scattered cells of the " pleuro- 

 peritoneal epithelia," develop themselves periodically into 

 sexual cells. An independent differentiation of special sexual 

 organs seems, therefore, to occur later, perhaps at different 

 times in the different groups of animals. The decision of this 

 very difficult question is, in general, connected with the 

 problem of the homology of the sexual organs, and with the 

 primary phyletic origin of the sexual cells, one of the most 

 difficult problems of ontogenesis and phylogenesis. 1 would 

 in addition here to the observations which I have made on 

 this subject in the ' Biology of Calcareous Sponges ' (pp. 469, 

 471), wish to hint as to the possibility of both primary 

 germ-lamellse sharing in the formation of sexual cells. For, 

 although in most cases the origin of the sexual cells from 

 cells of the intestinal fibrous layer, or even of the primary 

 gastral layer, is proved, yet in other cases they appear to 

 originate just as certainly from the dermal muscular layer, or 

 even from the primary dermal layer (Hydra). 



On account of the positiveness with which opposite views 

 concerning the origin of the sexual cells are maintained even 

 within the single group of Zoophyta, it may finally still be 

 suggested whether a translocation of them has not occurred 

 so early (already within the Laurentian period) that their 

 apparently original conditions may now, indeed, be their 

 second home. I have proved that in the calcareous 

 sponges the egg-cells which originally arise in the endo- 

 derm often pass very early into the exoderm by their 

 amoeboid movements, and there continue their growth. 

 In many Calcispongise the egg-cells are much easier to be 

 found in the exoderm (their secondary place of abode) 

 than in the endoderm (their primary original position), 

 so that I even believed at one time that they arose origi- 

 nally in the former. We may now, perhaps, venture to 

 suppose that this early transport of the cells from one 

 primary cell-layer to the other, by continued " shortened or 

 contracted inheritance,^' in the course of generations, would 

 be continually thrown further back in ontogenesis, till it 

 finally takes place already during the differentiation of similar 

 furrowed cells into the two forms of cells of the two primary 

 germ-lamellee. Then the cells which originally (phylo- 

 genetically) belonged to the inner germ-lamellse nevertheless 



