610 E. W. MACBRIDE. 



EXPLANATION OF PLATES 43—45, 



Illustrating Mr. E. W. MacBride's paper on " The Early 

 Development of Araphioxus." 



List of Abbreviations employed. 



Al. Alimentary canal. Ch. Notocliord. End. Endostyle. gl. Club-shaped 

 gland. Muse. Longitudinal muscles of the somites. N. c. Nerve-cord. 

 Neph. Persistent communication of the left collar cavity with gut, commonly 

 called " Hatschek's nephridium." V. Coll. Ventral portion of collar cavity. 

 V. Tr. Trunk body-cavity produced by the fusion of the ventral portions of 

 the somites, x marks the dorsal lip of the blastopore in gastrulse. 



The last four figures are diagrams ; the outlines of all the rest have been 

 drawn with the camera lucida. 



PLATE 43. 



(All the figures drawn with magnification obtained by Zeiss C, oc. 2.) 



Pig. 1. — Section of young blastula. 



Fig. 2. — Sagittal section of blastula which is just commencing to flatten. 



Fig. 3. — Sagittal section of blastula which has flattened on one side. 



Fig. 4. — Sagittal section of gastrula in which invagination is just com- 

 mencing. 



Fig. 5. — Sagittal section of gastrula in which invagination is more advanced. 



Fig. 6.— Sagittal section of gastrula in which invagination is still more 

 advanced. 



Fig. 7. — Sagittal section of gastrula in which invagination is well advanced ; 

 the first trace of the flattening which marks the dorsal side is visible. 

 ■ Fig. 8. — Sagittal section of gastrula in which invagination is very advanced. 



Fig. 9. — Sagittal section of gastrula in which blastopore is becoming 

 narrowed. 



Fig. 10. — Sagittal section of completed gastrula. The blastopore has been 

 shifted to the dorsal side (in consequence of a slight obliquity a piece of 

 the medullary fold is included in the section). 



Fig. 11. — Transverse section of embryo about the age of that in Fig. 10 

 (7 — 8 hours from fertilisation). 



Fig. 12. — Transverse section of embryo of about ten hours ; first trace of 

 neural plate and ccelomic groove. 



