302 SUMMARY OF CURRENT RESEARCHES RELATING TO 



teron appears to b3 formed by an invagination similar to that found in 

 Urodela and Petromyzon. 



Compared with Ceratodus, the medullary folds are extremely low, 

 and in the trunk region merge into one another, the arching over being 

 effected by superficial cells only. The cavity so enclosed soon vanishes, 

 and the central canal of the nervous axis arises secondarily. There is 

 no neurenteric canal, and the blastopore early becomes the cloacal 

 opening. 



Four pairs of external gills are developed upon the four branchial 

 arches, and are apparent as four prominent knobs about four days before 

 hatching. By this time a crescentic cement organ, corresponding to 

 that of the larva of Anura, has appeared. Like the central canal of the 

 nervous axis, ear, mouth, and nose cavities are formed by secondary ex- 

 cavations in an originally solid rudiment. 



The newly hatched larva, of a pale yellowish salmon colour but 

 unpigmented, is superficially tadpole-like. The " blastopore " closes 

 about the time of hatching for about two weeks ; but for some time 

 previous to this the pronephric duct has completed its growth and 

 has established its opening into the hind-gut. In the second week 

 pigment appears, first in the pigment-layer of the retina, and soon 

 afterwards on the dorsal surface of the head and anterior body region. 

 The external gills are now pinnate, and growing rapidly, and when 

 fully formed are raised upon a common stalk. While the external gills 

 are still at their full development, pulmonary breathing has commenced, 

 and the branchial clefts become perforate long after they are com- 

 pletely overgrown by the opercular fold. The external gills are lost 

 about six weeks after hatching. By differential growth they come to 

 lie directly above or behind the root of the fore limb. The fore and 

 hind limb undergo, as in Ceratodus, a rotation in opposite directions. 



The young Lepidosiren is remarkably newt-like. It shows a colour 

 change with varying light and darkness. 



Merogonic Fertilisation.* — Prof. Yves Delage takes account of some 

 of the comments which have been made on his researches on merogonic 

 fertilisation. He points out how his work differs from that of the 

 Hertwigs and Boveri. He rejects Giard's interpretation that the phe- 

 nomena were those of male parthenogenesis. He reasserts his convic- 

 tion that the essential phenomenon of fertilisation is the substitution of 

 a male nucleus for a female nucleus in the cytoplasm of the ovum, the 

 male nucleus having an excitability which the inert female nucleus has 

 lost, and which leads to division. 



Convergence.! — Herr Philippi expresses a strong belief in the law 

 of recapitulation, but he admits that its recognition is not always easy. 

 Coenogenesis is one source of obscurity, or rather one of the ways in 

 which we interpret cases where the recapitulation-theory fails to fit. 

 Convergence is another stumbling-block ; there may be superficial re- 

 semblance between entirely distinct types, e.g. blindworms and burrow- 

 ing snakes, or within the same order or family, e.g. the occurrence of 

 the Mytilus-form in Myalina, Myoconcha, and Mysidioptera. Again, 



* Arch. Zool. Expe'r., vii. (1899) pp. 511-27. 



t SB. Gea. Naturfreunde Berlin, 1899, pp. 87-90. 



