612 E. W. MACBRIDB. 



c. Shows backward continuation of the ventral part of the collar cavity 

 {V. Coll.). V. Tr. A solid wedge of tissue which represents the trunk 

 coelom formed by the fusion of the ventral ends of the somites. 



Tig. 20, a, b, and c. — Tliree sections from a series through an embryo of the 

 same age as that figured in Fig. 19, showing the dying out of the ventral 

 extension of the collar cavity {V. Coll.). 



Fig. 21, a, b, and c. — Three sections from a series through a pelagic larva 

 showing relations of the fully developed nephridiura of Hatschek. 



a. Shows the opening into the cavity of the first muscular somite (left 

 collar cavity). 



b. Shows appearance of a section across the middle of the structure. 



c. Shows tlie opening into the pharynx. 



PLATE 45. 



Fig. 22, a, b, and c. — The ventral portions of three sections through an old 

 pelagic larva, showing the formation of the atrial fold and the continuation of 

 the ventral part of the collar cavity into it. gl. Club-shaped gland. 



Fig. 23, a and b. — The ventral portions of two sections through a still 

 older pelagic larva, showing a further stage in the development of the atrial 

 folds. 



Fig. 24. — Ventral portion of a section through a young adult Amphioxus, 

 showing the cavities of the metapleural folds (the same as the collar cavities), 

 and the schizocoelic artefacts which appear to the outer side of them. 



Fig. 25. — Diagram showing the origin of the various portions of the 

 ccelom from the gut in Amphioxus. 



Fig. 26.— Similar diagram to show the origin of the ccelom in Balano- 

 glossus. 



Fig. 27. — Diagrammatic lateral view of an Amphioxus larva, showing 

 mutual relationships of the head cavities, collar cavities, and trunk ccelom. 



Fig. 28. — Similar diagram of Balanoglossus. 



(Fig. 22, «, b, and c are drawn with Zeiss F, oc. 2 ; Figs. 23 and 24 

 with Zeiss D, oc. 2). 



N.B. — In Figs. 17 to 20 the hypoblast and mesoblast are coloured red, and 

 the epiblast grey. 



On the contrary, in Figs. 21 to 24 the epiblast is coloured red, the meso- 

 blast blue, and the hypoblast, including the notochord, grey. 



