100 SUMMARY. 



segmentation, (2) unequal segmentation. These two being exter- 

 nally similar to the similarly named types in the first group. 

 (3) Superficial segmentation. This is unlike anything which is 

 present in the first group, and is characterized by the appearance 

 of a superficial layer of cells round a central yolk mass. These 

 cells may either appear simultaneously or successively, and their 

 nuclei are derived from the segmentation within the ovum of the 

 first segmentation nucleus. 



The types of ova in relation to the characters of the segmentation 

 may be tabulated in the following way: 



Segmentation. 



(1) alecithal j ^ 



ova J 



(2) telolecithal ) (a) unequal 



ova J (6) partial 



(3) centre- \ ^ re ular ( witl1 segments united in 

 ^ ' i -,i i f central yolk mass) 



lecithal \ n\ i 



ova f W unequal 



UVcl I i \ J2 ' 1 



(c) superficial. 



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 embryological 

 repetition of that phase in the evolution of the Metazoa, which consti- 

 tuted 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 segmentation has great 

 influence on the early phenomena of development, though naturally 

 none on the adult form. 



EXTERNAL FEATURES OF SEGMENTATION. 



(10=;) E. Haeckel. "Die Gastrula u. Eifurchung." Jenaische Zeitschrift, Vol. ix. 

 1877. 



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



u. n. ihrer Bedeuturig." Oken Isis. 1848. 



