160 Development of Lepidosiren paradoxa, Fitz. 



" The External Features in the Development of Lepidosiren 

 paradoxa, Fitz. By J. Graham Kerr. Communicated by 

 A. Sedgwick, F.E.S. Eeceived April 11, — Eead May 4, 

 1899. 



(Abstract.) 



The paper opens with a short account of the habits of Lepidosiren as 

 observed in the Gran Chaco. A description is then given of the 

 "external features in the development. The more important points in 

 this may be summarised as follows. 



The egg is very large, 6 -5 — 7 mm. in diameter. It is surrounded 

 by a special capsule at first thick and almost jelly-like in appearance, 

 later on (after fertilisation) thin and horny. Outside this was found in 

 rare cases a thick jelly resembling that of the common frog's egg. 

 The egg is without a trace of dark pigment. Segmentation is com- 

 plete, resembling most nearly that of the egg of Amia, and leads to a 

 condition with an upper hemisphere of small cells with large segmenta- 

 tion cavity, and a lower of large yolk cells. Gastrulation begins with 

 the appearance of a row of depressions, or a continuous groove along 

 about one-third of the whole extent of the margin between small and 

 large cells. During its progress the small-celled portion spreads over 

 the lower yolk cells by the addition to its margin of small cells split 

 off from the yolk cells. As the groove referred to deepens into a slit 

 to form the archenteron, it becomes gradually shorter, and the 

 eventual complete blastopore is a crescentic slit only about a quarter of 

 the length of the original groove. The medullary folds soon appear 

 running forwards from the blastopore. There is no trace externally of 

 a blastoporic or protostomcd seam running along the back between the 

 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 

 amphibian-like. It possesses large pinnate external or somatic gills, 

 four on each side, corresponding to branchial arches I, II, 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. 

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

 retina, then ove^ the dorsal surface, especially anteriorly. The larval 

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

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



