122 SUMMARY. 



Although the various types of segmentation which have been 

 described present very different aspects, they must nevertheless 

 be looked on as manifestations of the same inherited tendency 

 to division, which differ only according to the conditions under 

 which the tendency displays itself. 



This tendency is probably to be regarded as the embryologi- 

 cal repetition of that phase in the evolution of the Metazoa, 

 which constituted the transition from the protozoon to the 

 metazoon condition. 



From the facts narrated in this chapter the reader will have 

 gathered that similarity or dissimilarity of segmentation is no 

 safe guide to affinities. In many cases, it is true, a special type 

 of segmentation may characterize a whole group ; but in other 

 cases very closely allied animals present the greatest differences 

 with respect to their segmentation ; as for instance the different 

 species of the genus Gammarus. The character of the segmen- 

 tation has great influence on the early phenomena of develop- 

 ment, though naturally none on the adult form. 



EXTERNAL FEATURES OF SEGMENTATION. 



(105) E. Haeckel. "Die Gastrula u. Eifurchung." Jenaische Zeitschrift, 

 Vol. IX. 1877. 



(106) Fr. Leydig. "Die Dotterfurchung nach ihrem Vorkommen in d. 

 Thierwelt u. n. ihrer Bedeutung." Oken his. 1848. 



