1884.] 157 [Hyatt. 



and also to sustain other conclusions cited above, with regard to 

 the effects of growth in the primitive production of such organs, 

 especially among the ancestors of the Vertebrata. 



Dr. Hatscheck (Arb. Zool. Inst. Wien., vol. iv, 1881, p. 45-48) 

 attributes the origin of the primitive segments and other changes 

 of form in the embryo of Amphioxus to the growth and energy of 

 cells. He explains the origin of the medullary plate by differen- 

 tiations in the cells caused by the extra growth of the neighbor- 

 ing cells of the ectoderm, and attributes the rise of the ends and 

 final inclosure of the neural canal to lateral outgrowths due to the 

 same cause. 



The general presence of the different forms of the gastrula, in- 

 cluding the planula, indicates, as we have tried to show above, 

 that Haeckel was right in supposing that these stages indicated 

 common ancestors for the whole animal kingdom. To this we 

 have also joined (page 87) the architroch of Lankester by imagining 

 a very ancient origin for the circles of oral cilia, around the blas- 

 topore of the primitive gastrula- like ancestors of the Invertebrata. 



The history of the structural transitions through which the layers 

 of the body pass in their subsequent history sustains the view that 

 the Porifera are the lowest type of Metazoa. The endoderm and 

 ectoderm reach a highly differentiated stage and appear as flat 

 epithelial membranes, but the middle layer remains a mesenchyme 

 containing, as we have stated above, the reproductive bodies of 

 both sexes. The appearance of spermatozoa and ova indifferently 

 in the same animal shows, that entire separation of the sexes does 

 not take place so far as now known, among Porifera. It is not 

 yet established, that cross fertilization occurs in any form, though 

 there are as yet no grounds for the positive assertion that it does 

 not occur. The history of the earty stages exhibits a larval form 

 in which the interior is solid for a certain period and the mesen- 

 chyme plays a much more important role than in any other branch 

 of the animal kingdom, as might be anticipated from the adult 

 condition and importance of this layer in the morphology of the 

 group. 



We have also tried to show that the general morphology and 

 development indicated the gradual evolution of series of forms 

 from a type similar to Ascones but without a skeleton, which we 

 have considered directly comparable, as stated by Haeckel, with the 



