Dr. W. Saleiusky on IliickeVs Gastraja Theory. 15 



tigations of Kowalevsky*. The scorpion also presents similar 

 conditions, according to the researches of Alecznikofff. 



If we "wish to snni up the various ontogenetic phenomena, 

 draw conclusions as to the developmental processes from ob- 

 servations, and establisli these as the basi^for our subsequent 

 observations, we must, in the tirst place, select the most im- 



f)ortant phenomena common to all animals in the developmental 

 listory of their organization, and distinguish these from the 

 secondary plienomcna, wliicli are manifested later and in a 

 difterent 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 {zusammenlagern) 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 di-aw 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 difterentiation 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 -lamellae are 

 essential for all organisms, because they appear first of all in 

 all animals, and lay a foundation for further organic develop- 

 ment. 



III. General review of the first emhryological processes of 

 Animals. 



In order to place ourselves in a right position with regard 

 to the general embryological processes, we must commence from 



* Memoires de I'Acad. de St. Petersb. tome xvi. 

 t Zeitschr. fiir wiss. Zool. Bd. xxi. 



