Larval Theory of the Origin of Tissue. 205 



and attributes the rise of the ends and final enclosure of the 

 neural canal to lateral outgrowths due to the same cause *. 



The general presence of the different forms of the gastrula, 

 including the planula, indicates, as we have tried to show 

 above, that Hiickel was right in supposing that these stages 

 indicated common ancestors for the whole animal kingdom. 

 To this we have also joined the architroch of Lankestcr, 

 imagining in common with this author a very ancient origin 

 for the circles of cilia around the blastopore 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 stated by all authors, 

 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 the Porifera. It is not yet established 

 that cross-fertilization occurs in any form, though there is as 

 yet no ground for the positive assertion that it does not occur. 

 The history of the early stages exhibits a larval form in wliich 

 the interior is solid for a certain period and the mesenchyme 

 plays a much more important role than in any other branch 

 of the animal kingdom, as might be anticipated from tlie 

 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 

 Hackel, with the gastrula. During this evolution the 

 mesenchyme became more and more important, and as a 

 result of its thickening the habit of budding was more or less 

 suppressed, so that the higher tyj^es must be considered as 

 individuals with a highly plastic form, liable to excessive 

 outgrowths, but not as branching Metazoons. The archen- 

 teron also remains persistent throughout life, gives rise to 

 simple diverticula, or, in forms with thick mesenchyme, 

 diverticula themselves ibrm branching tubes. 



The fact that no internal column or body-cavity is formed, 



* See also His, ' Unsere Korperform,' 1875, pp. 60, 61,83, and 178, who 

 has essentially the same idea of the relations of growth of cells and deve- 

 lopment of organs. 



