188 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Development of Central Nervous System of Amphibians.* — Dr. H. 

 Orr finds that the central nervous system of Amphibians first appears 

 as a transverse epiblastic thickening dorsal to the mouth-fusion, and con- 

 tinues with paired elongated epiblastic thickenings lying dorsally on 

 either side of the median line. The primary cranial flexure is due to 

 the presence of this transverse epiblastic thickening or anterior medullary 

 plate. When the brain is inclosed this thickening forms that part of 

 the brain-wall which lies between the infundibulum and the optic 

 groove. The first nerve- fibres which develope in the brain appear on 

 what was originally the internal surface of the primitive epiblastic 

 thickenings which run longitudinally in the dorsal region and unite 

 continuously in the region of the primitive transverse thickening. A 

 subsequent development of nerve-fibres gives rise to a continuous ventral 

 commissure which extends through the floor of the mid- and hind- 

 brain and of the spinal cord, as well as to the anterior and posterior 

 commissures of the brain. The fibres of the optic nerves are intimately 

 connected with and are developed in the same manner as the main bundle 

 of fibres in the region of the primitive transverse epiblastic thickening. 

 The mode of development of the hypophysis of Amblystoma has been 

 studied, and it has been found to be intermediate between that of the 

 lizard and that of the frog. 



The structure of the larval rod-like organs which Clarke called 

 " balancers " in Amblystoma has been investigated, and they have been 

 found to be homologous with external gills, so that we have the case of 

 a homologue of the external gills being metamorphosed into an organ 

 for the support of the body. It is possible that further research will 

 show that the suckers on the tadpole of the frog are similar organs. 



Protandric Hermaphroditism of Myxine.t— Mr. F. Nansen finds 

 that Myxine glutinosa is, ordinarily, a protandric hermaphrodite. Till 

 its body is about 32 cm. long it is a male, and after that it produces ova. 

 The proportions of the male and female portions of the gonad are not 

 constant, but the male is generally one- third of the whole length of the 

 organ. In a few cases what are called " true " males were observed, but 

 they are probably transformed hermaphrodites. This strange irregularity 

 in the occurrence and extent of the male and female organs seems to 

 show that Myxine is an animal which, in sexual respects, is just at 

 present in a transition stage ; it seems still to be seeking, without yet 

 reaching, that mode of reproduction which is most profitable for it in 

 the struggle for existence. 



The young testicular follicles are similar in structure to the young 

 ovarian ; they contain a large sexual cell, spermatogon, which is enveloped 

 by an epithelium, follicular epithelium, and a connective-tissue envelope. 

 This spermatogon undergoes subdivision and becomes converted into 

 spermatides which are separated from one another and swim in a fluid 

 contained in the testicular capsules. The nucleus and whole cell 

 gradually elongate, and, on the bursting of the testicular capsules, ripe 

 spermatozoa pass into the body-cavity. Mr. Nansen does not agree with 

 Mr. J. T. Cunningham's account of the form of the spermatozoa, and 

 thinks that observer's specimens must have been in some way altered. 

 Nearly ripe spermatozoa may be found in specimens of Myxine at all 



* Quart. Journ Micr. Sci., ssix. (1888) pp. 295-324 (3 pis.). 



t ^ereren's Museum Aaraberetning for 1887 (1888) No. 7, 34 np. (2 pis.). 



