22 



PERIPATUS 



of the somites is shown in Tig. 14, A, which represents a transverse 

 section taken between the mouth and anus of an embryo of the 

 stage of Fig. 13, D. The history of these somites is an exceed- 

 ingly interesting one, and may be described shortly as follows :- 

 They divide into two parts a ventral part, which extends into 



Fig. 14. A series of diagrams of transverse sections through .Perijmtus embryos to 

 show the relations of the coelom at successive stages. (After Sedgwick.) A, Early 

 stage : 1, gut ; 2, mesoblastic solnite ; no trace of the vascular space ; endoderm 

 and ectoderm in contact. B, Endoderm has separated from the dorsal and ventral 

 ectoderm. The somite is represented as having divided on the left side into a 

 dorsal and ventral portion : 1, gut ; 2, somite ; 3, haemocoele. C, The haemocoele 

 (3) has become divided up into a number of sj)aces, the arrangement of which is 

 unimportant. The dorsal part of each somite has travelled dorsalwards, and now 

 constitutes a small space (triangular in section) just dorsal to the gut. The ventral 

 portion (2') has assumed a tubular character, and has acquired an external opening. 

 The internal vesicle is already indicated, and is shown in the diagram by the thinner 

 black line : 1, gut ; 2', nephridial part of coelom ; 3, haemocoele ; 3', part of 

 haemocoele which will form the heart the part of the haemocoele on each side of 

 this will form the pericardium ; 4, nerve-cord. D represents the conditions at 

 the time of birth ; numbers as in C, except 5, slime glands. The coelom is re- 

 presented as surrounded by a thick black line, excejit in the part which forms the 

 internal vesicle of the uephridium. 



the appendage, and a dorsal part (Fig. 14, B). The ventral part 

 acquires an opening to the exterior just outside the nerve-cord, 

 and becomes entirely transformed into a nephridium (Fig. 14, 

 D, 2^. The dorsal part shifts dorsalwards and diminishes rela- 

 tively in size (Fig. 14, C). Its fate differs in the different parts 



