INTRACELLULAR DIGESTION OF INVERTEBRATES. 107 



Koch has found both Bacillus anthracis and the bacillus of 

 septicsemia in the mouse enclosed by white blood-corpuscles/ 

 while tubercle bacilli have been seen by him in the interior 

 of giant cells.2 So that throughout the whole animal kingdom 

 the wandering cells of the mesoderm make use of their inges- 

 tive power for the destruction of bacteria and similar organisms^ 

 which need for their development a suitable (necrotic) nidus. 

 In Metazoa with an undeveloped mesoderm, this function is 

 performed by the ectoderm (Plumularia) or by the endoderm. 



Most embryologists agree that the Metazoa are to be derived 

 from ancestors closely resembling the colonial Monads. Now, 

 the individuals composing these colonies are all exactly similar 

 one to another, so that we can find among them no trace of 

 that division of labour which is the first step in the differen- 

 tiation of germinal layers. There is even less differentiation 

 among the colonial Protozoa than among their chlorophyll con- 

 taining allies, the Volvociueae. The attempt to form some exact 

 idea of the origin of the lowest Metazoa, as the basis of a com- 

 parative biology, is, therefore, extremely difficult. Observers are 

 agreed in considering the blastula stage to represent an an- 

 cestral colony of Monads ; but they diff'er fundamentally in 

 the meaning attached to the formation of the germinal layers. 

 Some, as, for example, Balfour, unhappily so early lost to 

 science, suppose that the blastula cells (or rather, the individuals 

 composing the colony which this stage represents) were early 

 differentiated into two kinds; so that the transitional form 

 between Protozoa and Metazoa consisted of a hemisphere of 

 nutrient amoeboid cells, joined to a hemisphere of ciliated 

 locomotive cells.^ I have supposed that some of the blastula 



' ' Unters. ueb. d. Aetiologie d. Wundinfectionskrankheiten,' 1878, pp. 44, 

 72. Koch's opinion that bacteria force themselves into the white corpuscles 

 in order to multiply there does not seem proved, and does not agree with my 

 results. 



2 ' Berliner kliuische Wochenschrift,' 1882, No. 15. Zopf's interpretation 

 of Koch's observations does not seem probable (' Die Spaltpilze,' Berlin, 1883, 

 p. 67). 



' " . . . The larva of Sponges is to be considered as a colony of Protozoa, 



