12 INTRODUCTION 



There are a number of animals in which the mesoderm 

 produces, in addition to the specific organs that have arisen 

 from it (genital organs, excretory organs), only mesenchyma. 

 Such is the case in the Platyhelminthes. In the great 

 majority of the Bilateria, on the contrary, the formation of 

 mesenchyma and coelom occurs together, and there is there- 

 fore a sort of competition between the two methods of 

 mesoderm development, so that in one case (Annelids, Sagitta, 

 Phoronis) the formation of a ccelom predominates, in the 

 other (Mollusca, Arthropoda) that of a pseudocoele (mesen- 

 chyma). 



In the Bilateria there arise from mesoderm the muscula- 

 ture, the genital organs,^ the excretory organs known as 

 nephridia, the connective tissue, and fatty tissue. 



1 [The statement that the fundament of the genital organs of the 

 Bilateria comes from the mesoderm ought to be considerably restricted. 

 In recent times the observations have been increasing which appear to 

 support the Weismannian doctrine of the continuity of the germ cells. 

 Grobben some time ago observed the early differentiation of the sexual 

 cells in Moina. The same has been known for a long time to be true 

 of Diptera and Aphidse. Recently Heymons {Sitz.-Ber. Gesell. Nat. 

 Frennde, Berlin Jahrg., 1893, p. 263) has found similar conditions in 

 various Orthoptera. The investigations of Faussek on Phalangidae [Biol. 

 Centralbl., Bd. xii., p. 1, 1892) and the very recent observations of A. 

 Bkauer {Zeitschr. f. wiss. Zool, Bd. Ivii. 1894) on the scorpion have 

 shown the early independence of the genital fundaments in these forms. 

 Special importance in the present question' is to be attached to the 

 observations of Boveei on Ascaris (Sitz.-Ber. Gesell. f. Morph. u. Physiol. 

 Milnchen, Bd. viii., 1892), according to which the sexual cells are dis- 

 tinguished from the somatic cells even in the first stages of cleavage 

 owing to the special structure of the chromatic elements of their miclei.] 



