152 WHITMAN. [Vol. I. 



" MetschnikofTs inneres Blatt des Keimstreifens allein ist das 

 Mesoderm, dessen Betheiligung am Aufbau des Bauchmarks er 

 nicht gekannt hat ; sein ausseres Blatt des Keimstreifens gehort 

 dem Ectoderm an, und er hat hier ganz richtig dessen Theilnahme 

 an der Bildung des Nervensystems erkannt, dagegen seine erste 

 Entstehung aus dem ectoderm nicht beobachtet." 



According to Semper, Metschnikoff and Kowalevsky are wrong 

 only in supposing that the whole of the nerve-chain arises from 

 the ectoderm, while Rathke was equally wrong in deriving it 

 wholly from the mesoderm. Semper (p. 295) extends the idea 

 of a double origin of the central nervous system to all segmented 

 animals. The median part (" centrale Ganglionzellenstrang"), 

 which is supposed to arise as an unsegmented cord, from a 

 thickening of the ectoderm, is regarded as homologous with 

 the spinal cord of vertebrates ; the lateral ganglia, arising in 

 the manner described by Rathke from the already segmented 

 mesoderm, are homologized with the spinal ganglia of ver- 

 tebrates.^ 



Hatschek's remarks (Nos. 24, 25) on the origin of the 

 nervous system in the Hirudinea are of an incidental character, 

 and are of interest only on account of the general scheme set 

 up for the annelids. The ventral nerve-chain, according to 

 Hatschek, arises from three rudiments, one median and two 

 lateral. The median rudiment is a longitudinal infolding of 

 the ectoderm along the ventral line, and is marked with a 

 groove like the medullary groove of vertebrates. The lateral 

 rudiments are prolongations of a pre-oral, neural plate (" Schei- 

 telplatte "), which had its origin in an unpaiired thickening of 

 the ectoderm. 



The " neural groove," which forms the most seductive feature 

 of Hatschek's scheme, has nothing whatever to do with the for- 

 mation of the ventral nerve-chain, as has been clearly shown by 

 Kleinenberg. In concluding his excellent review of Hatschek's 

 observations, Kleinenberg (No. 26, p. 123,) states the case thus: 



^ In a foot-note (p. 179) Semper admits that the spinal ganglia may originate in 

 the neural plate, as maintained by Balfour; but, in this case, he would still maintain 

 their homology with the lateral ganglia of annelids. 



(24.) Hatschek, B. Beitrage zur Entwickelungsgeschichte und Morpholo- 

 gie der Anneliden. Sitzb. Akad. Wiss. in Wien. LXXIV. 1876. 



(25.) Studien iiber Entwicklungsgeschichte der Anneliden. Aheiten a. a. 

 zool. Inst, ztt Wien. I. 3. 1878. 



