170 



INVERTEBKATA 



CHAP. 



necessary to give a brief account of the principal points elucidated 

 by Professor Sedgwick, in order that we may have a correct appre- 

 hension of the significance of the development of other Arthropoda. 



PROTOTRACHEATA (ONYCHOPHORA) 



The embryo of Peripatus capensis passes through its entire 

 development within the oviduct of the mother, and it is born in a 

 form in which it already exhibits all the essential features of the 

 adult. The egg is very minute and, like the eggs of many Insecta, 

 is of an elongated ellipsoidal shape, its longest axis measuring about 

 4 mm. It is telolecithal, that is to say that there is a darker area 

 with a minimum of yolk and a maximum of cytoplasm situated at 

 one end of the shortest axis of the ellipsoid, whereas the rest of the 

 egg is paler in colour and richer in yolk. Segmentation is complete ; 

 the first four segments formed by the first two cleavages are of the 

 same size, but the third cleavage separates off four smaller darker 

 " animal " cells from four larger paler " vegetative " cells. The first- 

 named by rapid division 

 id gi ye r i ge t th e ectoderm, 

 the latter to the endo- 

 derm. 



When segmentation 

 is completed, the endo- 

 derm consists of a num- 

 ber of larger cells loosely 

 connected one with 

 another by strings of 

 cytoplasm which occupy 

 most of the space within 

 the egg membrane, the 

 ectoderm on the other 

 hand forms a cap of 

 small closely aggregated 

 cells which are also 

 connected together by 

 filaments of cytoplasm 

 (Fig. 123 A). 



Sedgwick justly at- 

 tached considerable im- 

 portance to these con- 

 necting filaments, and 

 held that they upset 

 the popular conception 

 of a cell as an isolated unit, and of a Metazoon animal as a 

 mere collocation of such units, or as a colonial Protozoon. He was 

 inclined to regard a multinucleate Protozoon, such as Actinosphaerium, 

 as giving a better idea of the common ancestor of Metazoa. Most 



end 



FIG. 123. Stages in the segmentation and the gastnila- 

 tion of the egg of Peripatus capensis. (After 

 Sedgwick.) 



A, conclusion of segmentation. The ectodermic c^lls form a 

 small cap resting on the endoderm cells which are loosely dis- 

 persed within the egg membrane. B, the endodermic cells are 

 contracted so as to form a compact mass, and the ectodermic cells 

 have begun to grow over them (epibole). C, the covering-in of 

 the endoderm cells is almost complete a few endoderm cells 

 protrude through the blastopore. ect, ectodermal blastomeres ; 

 end, endodermal blastomeres. 



