506 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



of development through a diploblastic stage which he termed the 

 planula. Such a planula stage he regarded as a repetition of a 

 common ancestral stage of evolution. 



Haeckel (1872) about the same time as Lankester also developed 

 the idea that the diploblastic stage of ontogeny was to be interpreted 

 as the repetition of an ancestral form : Haeckel called this ancestral 

 form Gastraea. The main difference between Haeckel's view and 

 Lankester's was that the former regarded the endoderm as having 

 arisen by a process of invagination as it actually does arise in 

 ontogeny in the great majority of cases while Lankester regarded 

 it as having arisen by a process of delamination from the outer 

 layer. 



As regards the middle germ-layer ideas remained somewhat vague 

 until Agassiz (1864) showed that in the Starfish the mesoderm arose 

 in the form of an outgrowth of the archenteric wall. The same was 

 found to be the case in various other Invertebrates, and in 1877 

 Kowalevsky showed how in Amphioxus the mesoderm was during an 

 early stage in the form of archenteric pockets. In the same year 

 Lankester developed the generalization that the coelome is to be 

 regarded as uniformly enterocoelic in origin and comparable with the 

 diverticula of the archenteric lining seen in Coelenterata. 



The separation of such mesodermal cells as are in their early 

 stages free and amoeboid under the common name mesenchyme was 

 first made by O. and R. Hertwig (1882). 



The later developments of the theory of the mesoderm involved 

 in the Protostoma theory have already been alluded to earlier in this 

 volume and the same applies to what the author regards as the chief 

 qualification of the germ -layer theory indicated by modern work, 

 namely that the boundary between two layers where they are con- 

 tinued into one another must be regarded not as a sharply marked 

 line but as a more or less broad debatable zone. 



LITERATURE 



Agassiz. Contributions to the Natural History of the United States of America, v. 

 Boston, 1864. [Vol. v printed as vol. v, pi. 1, of Mem. Mus. Comp. Zoology 

 Harvard.] 



Baer. Ober die Kntu i< -kelungsgeschichte der Thiere. Beobachtung und Reflexion, 

 i. KniiigshtM-g, 1828. 



Bell. Arch. Kntuick. Mrchanik, xxiii, 1907. 



Fiirbringer. (Ii-gcnlKiurs Foi^-ln-il'l. Lrip/i^, 1897. 



Haeckel. !>! Kjuktohwamme. Berlin, 1872. 



Heriwig, O. Arch. mikr. Anat., xxxix, 1892. 



Hertwig, 0. and B. .li-uaischi- /i'it.vhrii'1, xv, 1882. 



Jdrgensen. li. I li-rt \\ij_- 1 . -t-.-hnit. .inui. 1910. 



Kerr, Graham. Pi<>r. \i',\. l'h\-. Sue. Ivlin.. xviii, 1912. 



Kopsch. Internal. ICowtiaohr, Anal, u. I'h\s., xvi, 1899. 



Kowalevsky. M.-m. A<M.|. s.-j. st Pftenbourg, (7), xvi, 1871. 



Kowalevsky. Arch. mikr. Anal., xiii, 1877. 



Lankester. Ann. Mag. Nt Ili-t., (4), xi, 1873. 



Lankester. <,>nart. .Imiin. Mii-r. Sri., \\ii, 1877. 



Lereboullet. Am, i, /,,!.. \x, 1863. 



