293 



medullary folds. The folds are low and inconspicuous, and they are 

 continued into one another behind the blastopore, which becomes the 

 anus. There are only slight traces of overarching of the medullary 

 folds to enclose a neural canal. During the later stages of intraoval 

 development, the posterior end of the body becomes much more con- 

 spicuously folded off the yolk than the head end. The Lepidosiren 

 hatches out as a tadpole-shaped larva, still completely devoid of dark 

 pigment. Just about the time of hatching the cloacal opening closes 

 temporarily. As the larva develops it becomes extraordinarily amphi- 

 bian-like. It possesses large pinnate external or somatic gills, four on 

 each side, corresponding to branchial arches I, IT, III, and IV. A large 

 cement organ is also present, which during its early stages is of the 

 characteristic crescent shape so usual in the embryos of Anura. Pig- 

 ment begins to appear about ten days after hatching — first in the 

 retina, then over the dorsal surface, especially anteriorly. The larval 

 condition lasts during the first six weeks after hatching. Towards the 

 end of this period the cement organ undergoes atrophy. The somatic 

 gills atrophy later. During the process of their doing so, the Lepido- 

 siren passes through a condition in which the stumps persist evidently 

 corresponding to that Avell known in the young Protopferus, the group 

 of external gills with their common stalk having come by difi'erential 

 to be situated immediately above the fore limb. After the close of the 

 larval period the Lepidosirens become much darker in colour and more 

 lively in their movements. Young were obtained from the nest up to 

 a length of 60 mm. About this time the cornea begins to assume the 

 white unhealthy appearance that it has in the adult. In the young of 

 this size, small yellow spots appear, and in the young of 90 mm these 

 are conspicuous. Occasional yellow blotches persist in the young 

 Lepidosiren of eighteen months, but in the adult they disappear. 



The paper concludes with general remarks on the phenomena 

 described. The segmentation approaches most closely that of Ganoids. 

 The shortening up of the invaginating groove is considered to illu- 

 strate a process which has taken place in phylogeny in the i)assage from 

 the primitive holoblastic e^^ to the meroplastic condition. The con- 

 tinuity of the medullary folds behind the anus is adduced together 

 with the evidence accumulating of the prolongation of the blastopore 

 along the floor of the medullary groove in other forms (Amphibia, 

 Ceratodus^ e.g.) as affording potent evidence in favour of the hypothesis 

 which derives the Vertebrata from ancestral forms as primitive as the 

 Coelenterata, and possessing an elongated mouth traversing the neural 

 surface. The occurrence of external gills in the young of three so 

 comparatively primitive groups of Vertebrata as Crossopterygians, 



