350 EMBRYOLOGY 



came separated at once into a lai'ge number of segments 

 by a rearrangement of the individual organs. Tliis assump- 

 tion is supported by the consideration that with the lateral 

 sinuous movement of the body, and with the rigidity of the 

 tissues caused by increasing differentiation, the formation 

 of alternating regions of greater and less motility was of 

 considerable advantage to the individual, and rendered 

 possible a further elongation of the body. The first cause 

 for the appearance of metameric segmentation would then 

 be sought in the manner of locomotion and in mechanical 

 conditions. However, this latter view is not supported in 

 any way by embryology. 



Even though we have not been able to give a positive 

 decision on these difficult questions, yet it seems to us ap- 

 propriate, in the present state of knowledge, to indicate the 

 direction of future inquiry by which a possible solution of 

 the questions is to be sought. 



Literature. 



I. Ch^topoda and Archiannelida. 



1. Agassiz, a. On the Young Stages of a Few Annelids. Annals 



Lyceum Nat. Hist. New York. Vol. viii. 1867. 



2. Agassiz, A. The Embryology of Autolytus cornutus. Boston. 



Jour. Nat. Hist. Vol. vii. 1863. 



3. Beddard, F. E. On the Anatomy and Histology of Pleurochaeta 



Moseleyi. Trans. Roy. Soc. Edinhurcih. Vol. vii. 1883. 



4. Benham, W. B. Studies on Earthworms. Quart. Jour. Micr. Sei. 



Vols. xxvi. and xxvii. 1886 and 1887. 



5. Beegh, R. S. Untersuchungen über den Bau und die Entwicklung 



der Geschlechtsorgane der Regenwürmer. Zeitsehr. wiss. Zool. 

 Bd. xliv. 1886. 



6. Bergh, R. S. Die Entwicklungsgeschichte der Anneliden. A 



compilation. Kosmos. Bd. xix. 1886. 



7. Bergh, R. S. Zur Bildungsgeschichte der Excretionsorgane bei 



Criodrilus. Arbeiten zool. Imtitut. Würzhurg. Bd. viii. 1888. 



8. BüLOw, G. Ueber Theilungs- und Regenerationsvorgänge bei 



Würmern (Lumbriculus variegatus Gr.). Arch. Naturg. Jahrg. 

 xlix., Bd. i. 1883. 



9. Clapaküde, E., und Metschnikoff, E. Beiträge zur Erkenntniss 



der Entwicklungsgeschichte der Chätopoden. Zeitsehr. wiss. 

 Zool. Bd. xix. 1869. 



