472 Scientific Intelligence. 
erata suggested by Heckel.* A still more important point 
developed i 
plant the Type theory, and give us in its new system 
sed upon the homology of the embryonic layers and of the 
primitive digestive cavit weckel attempts, in his Gastrea 
ectoderm and entoderm, has been distinctly proved for Acalephs, 
Echinoderms, Polyps, Worms, Arthropods, Tunicates, Mollusks, || 
and finally for Amphioxus, the papers of Johannes tiller, Krohn, 
Agassiz, Kowalevsky, Sars, Allman, Clapardde, Kupfer, Metsch- 
nikoff and others, are too well known to need citation in this con- 
his later name of gastrula. But let us follow his su sequent 
steps and separate what is known from what is stated as known 
by Heckel. It is known that the planula consists of an entoderm 
) 
walls of this primitive cavity is, in their case at least, invariably 
formed by the ectoderm. It is known, on the other hand, that in 
Actiniz, in Worms, in Hydroids,§ this primitive digestive cavity 
is hollowed out of the inner yolk mass of the embryo, and has its 
* Heckel, E. Die Kalkschwamme, Berlin, 1872. 
| Heckel, E. Die Gastrea Theorie, Jenaische Zeitschrift, ix, 1874. 
Maclay, N. Mikulcho. Jen. Zeitschrift, iv, 1868. 
[ey E. Zur Entwickelungsgeschichte d. Kalkschwiamme. Zeits. f. 
Wiss. Zool., xxiv, 1874. 
. © erage E. R. On the primitive cell layers of the Embryo. Ann. Mag. N. 
_ Fol, H. Die erste Entwickelung d. Geryonideneies. Jen. Zeitsch., vii, p. 471. 
