Dr. W. Salensky on Hackel’s Gastrea Theory. 27 
cesses, and not judge of the ontogenetic facts merely from the 
opinion derived from the anatomical facts. If we desire to 
take an objective view, we cannot say that when two different 
modes of development “occur in very nearly allied forms ”’ 
they are of no consequence to us on account of their relation- 
ship*. If the phylogenetic fundamental law is correct, the 
relationship of animals must only be ascertained from ontogeny ; 
otherwise the idea of relationship, which is derived only from 
tectological facts, is a preconceived opinion. 
In now concluding my remarks, I hope in these few words 
to have furnished the factual evidence :— 
1. That the most important factor in the ontogeny of 
animals is the first differentiation of the germ-lamelle. 
2. That this differentiation commences in different animals 
at different periods of their development, and in most cases 
leads to the Planula-form, which occurs in all animals, 
either in the pure form (in most animals) or in a modified 
form (Vertebrata and some invertebrate animals), and even 
exists as free-living animal forms. In many instances the 
Planula-form may be overleaped and replaced by the Dz- 
blastula. 
3. That the development of the stomachal cavity is a later, 
secondary developmental phenomenon, which occurs in diffe- 
rent animals in different stages of development, and cannot 
take a place in the idea of the fundamental form of develop- 
ment. 
4, That therefore the Gastrula-form cannot be accepted as 
a fundamental form in the developmental history of all 
Metazoa; and, consequently, 
5. That the problematical form “ Gastrwa”’ cannot be 
accepted as the “ stock-form ”’ for the higher animal stocks. 
EXPLANATION OF PLATE V. 
Figs, 1, 2,3. Three developmental stages of the oyster (original); Ex. 
exoderm ; En. entoderm; V. velum; S. shell; MM. invagina- 
tion of the anterior intestine; D. intestine. 
Fig. 4, Diblastula of an Ascidian (from Kowaleysky : “ Weitere Studien,” 
&e., in Arch. fiir mikr. Anat. Bd. vii. Taf. x. fig. 5): Ea. exo- 
derm; Hn. entoderm. 
Fig. 5. Diblastula of Hydrophilus (from Kowalevsky : “Embryol. Studien,” 
&e., in Mém. de lAcad. de St. Pétersb. tome xvi, Taf. ix. 
fig. 20): Ex. exoderm; En. entoderm; Nd. nutritive vitellus. 
Fig. 6. Gastrula of an Ascidian (from Kowaleysky, /. c. Taf. x. fig. 6): 
Ex. exoderm; En, Entoderm. 
Fig. 7. Transverse section through the embryo of Hydrophilus (from 
Kowalevsky, /. c. Taf. ix. fig. 26): Ex. exoderm; En. entoderm. 
* Hiackel, Die Kalkschwamme, Bd. i. p. 467, 
