Dr. W. Salensky on Hiéckel’s Gastrexa Theory. 15 
tigations of Kowalevsky*. The scorpion also presents similar 
conditions, according to the researches of MecznikoffT. 
If we wish to sum up the various ontogenetic phenomena, 
draw conclusions as to the developmental processes from ob- 
servations, and establish these as the basis for our subsequent 
observations, we must, in the first place, select the most im- 
portant phenomena common to all animals in the developmental 
history of their organization, and distinguish these trom the 
secondary phenomena, which are manifested later and in a 
different manner. The developmental processes of all animals 
consist of a gradual differentiation of the cells first formed, 
which in many cases commences even at the time of segmen- 
tation. By the process of segmentation either similar or dis- 
similar cells are formed. The differences between the seg- 
mentation-cells may make their appearance in some animals 
even at the time of the binary division of the egg-cell, in others 
not until a much later period. This shows that the commence- 
ment of the differentiation occurs at different periods of de- 
velopment in different animals. The subsequent phenomena, 
however, maintain in different animals a similar and definite 
direction, consisting in the combination (zwsammenlagern) of 
the heterogenous cells into two or three layers. In these 
layers the cells are similar. At the conclusion of this first 
differentiation a definite body-form of the embryo may be pro- 
duced; from the comparison of these forms in different animals 
we draw conclusions as to whether this form is or is not com- 
mon to all animals. If it is common, it is of great importance 
to our general conceptions. If we can derive from this general 
form the subsequent phenomena of differentiation in the vari- 
ous animals, this form has a great phylogenetic value, because 
this diversity shows us the course of the different divergences 
from a common fundamental form. If we would be quite 
consistent in the consideration of ontogenetic phenomena, we 
must take these most important phenomena alone into con- 
sideration, without mingling them with other organs of later 
occurrence. The differentiations of the germ-lamelle are 
essential for all organisms, because they appear first of all in 
all animals, and lay a foundation for further organic develop- 
ment. 
Ill. General review of the first embryological processes of 
Animals. 
In order to place ourselves in a right position with regard 
to the general embryological processes, we must commence from 
* Mémoires de l’Acad. de St. Pétersh. tome xyi. 
+ Zeitschr. fiir wiss. Zool. Bd. xxi. 
