38 J. GRAHAM KERR. 



12. The first-formed meseucliyme arises from sclerotomic 

 outgrowths, assisted by proliferation to a slight extent from 

 the subchordal hypoblast. 



13. The solid neural keel arises by thickening of the deep 

 layer of the epiblast. 



14. The egg of Protopterus closely resembles in its 

 early development that of Le pi do sir en. 



15. The roof of the segmentation cavity is, however, 

 thicker. 



16. And the invagination groove appears about 20° nearer 

 the lower pole of the blastula. 



17. The early development of Lepidosiren and Proto- 

 pterus shows an extraordinarily close resemblance to that 

 of Amphibia Urodela; a close resemblance to that of Petro- 

 myzon; and an only slightly less close resemblance to that 

 of Ganoids. 



EXPLANATION OF PLATES 1—4, 



Illustrating Mr. Graham Kerr's paper on " The Development 

 of Lepidosiren paradoxa," Part II. 



As already pointed out, bj " Stage u" 1 mean tlie stage represented by fig. 

 fi of my previous paper on tlie " External Features in Development." For the 

 convenience of readers of tlie present paper I have copied and selected a 

 number of these figures in Text-fig. 1 (p. 8). Tiiis will, I hope, obviate the 

 necessity on the part of the reader of having to frequently refer to a separate 

 publication. 



Fig. 1. — Vertical section of egg of Stage S, showing chinks between the 

 micromeres. IV B 631. 



Fig. 2. — Vertical section through egg, showing tlie commencing formation 

 of the definite segmentation cavity, s, c. VII A 151. 



Fig. 3. — Vertical section through the upper part of an egg slightly older 

 than the last. The segmentation cavity has here begun to spread laterally. 

 XXIV A 21. 



