THE DEVELOPMENT OF I-EPIDOSIRENf PAIiADOXA. 



chinks. The extent of these cavities varies consider- 

 ably in different eggs of the same age^ the blastomeres in 

 some being more rounded^ in others less rounded and more 

 flattened against one another. The more rounded condition 

 of the blastomeres in the former case does not appear to be 

 associated with the nuclei being in a state of karyokinetic 

 activity, as has been asserted to be the case in other forms. 



Text-fig. 1, illustratinj^ the course of development of the Dipneumona. — 

 The stages are numbered in accordance with my earlier paper. Roman 

 numerals indicate figures of Protopterus (after Budgett, 'Trans. Zool. 

 Soc.,' vol. xvi). The remaining figures refer to Lepidosiren. In 

 figs. 16 — 24 the embryo is for convenience shown spread out in one plane 

 and viewed from the dorsal aspect. The magnification is slightly over 

 two diameters. br. Rudiment of external gills, c.o. Cement organ. 

 h.l. Rudiment of hind limb. pyi. Pronephros. 8. Egg during segmenta- 

 tion. 10. An early stage of invagination, the invagination groove stretch- 

 ing round about one third of the egg's circumference. 18. A later stage 

 of invagination, the large yolk-cells being now for the most part covered 

 in by small cells, xiir. Corresponding stage in Protopterus. 14. Egg 

 at the close of invagination, showing the cresceutic blastopore. 16. 

 Dorsal view of an embryo in which the medullary folds have just become 

 visible, diverging posteriorly to embrace the blastopore. 17. Later em- 

 bryo where the folds have met behind tiie blastopore, and are approximated 

 in the middle region of the embryo ; the rudiment of the pronephros is 

 now visible as a slight bulging ou either side. 19. The medullary folds 

 are nearly com|)letely fused; tiie branchial rudiment is visible as a bulging 

 in front of the pronephros; indications of the myotomes are seen between 

 the pronephros and the neural rudiment. 22. The branchial rudiment 

 has greatly increased in size, the 0[)tic rudiments are conspicuous, the 

 pronephric ducts have grown considerably backwards. 24. Embryo in 

 which the branchial rudiment has become completely segmented ou the 

 right-hand side ; tiie central cavity of the neural rudiment has appeared 

 as a dark shadow. 25. Side view of a slightly older embryo in its 

 natural position on tiie egg; the rudiments of the four external gills now 

 form distinct projections; the rudiment of the cement organ has appeared 

 ventrally. xxxv. Corresponding embryo of Protopterus. 28. Larva 

 three days after hatching. 31. Larva (thirteen days) in which the ex- 

 ternal gills have become pinnate, and the rudiments of the limbs have 

 appeared (anterior hidden by external gills). 35. Larva with external 

 gills at their maximum ; the cement organ, now in course of atrophy, is 

 seen beneatii the throat, xxxv. Corresponding larva of Protopterus. 

 3G. Young Lepidosiren with external gills i'l process of atrophy. 



