550 SUMMARY OF CURRENT RESEARCHES RELATING TO 



lateral appendages of the medullary tube, but tliey are not outgrowths 

 of it, but purely epidermal structures. The first rudiments give rise to 

 the dorsal roots and their ganglia, while the ventral roots do not arise 

 till later ; they are not, either, indcj^eudent outgrowths of the medullary 

 tube, but connections between it and the adjacent ganglia, which gra- 

 dually become drawn out into cords. The rami dorsales grow out from 

 the upper end of the ganglia, and the separation, therefore, into sensory 

 and motor fibres does not correspond with the development of the dorsal 

 and ventral roots. In addition to the spinal nerves, and independently 

 of their rudiments, the lateral nerve appears as an epidermal gaugliouic 

 mass which, later on, becomes connected with the root of the vagus, and 

 grows out horizontally backwards; there are also five ganglionic bodies 

 within the mesoderm or above the gill-pouches, which only secondarily 

 enter into connection with one another, and with the vagus ; they give 

 ofi" branchial branches. The whole peripheral nervous system does not 

 therefore arise as one, nor even from one and the same germinal layer. 

 The histogenesis of tJie nervous system of Petromi/zon is essentially 

 similar to that of the Amphibia ; the nerve-fibres and nerve-cells appear 

 separately, and only become connected secondarily. 



The formation of mesodermal segments is continued as far as the 

 most anterior end of the head ; as in the Amphibia, the head consists of 

 four mesodermal segments ; they give rise to the trigeminal, facial- 

 auditory, glossopharyngeal, and vagus nerves ; the hypoglossus is re- 

 garded as the first spinal nerve of the trunk. The eight gill-sacs are 

 homologues of the inner gill-sac of the anurous Amphibia ; the " enteric 

 gills " of the lamprey are therefore essentially distinct from the ordinary 

 " dermal gills " of Fishes and Amphibians. 



The heart is developed behind the branchial region below the 

 oesophagus, so that the pericardial cavity communicates superiorly with 

 the coelom ; the endocardium is formed by the endoderm, and the blood 

 is formed in the ventral endoderm behind the rudiment of the liver. 

 In correspondence with the position of the heart the pronephros lies 

 exactly above the pericardiac cavity. 



Egg-shell of Lepadogaster.* — M. F. Guitel has investigated the 

 mode of attachment of the eggs of Lepadogaster. With moderate mag- 

 nification a small clear circle surrounded by a dark zone may be seen at 

 the centre of the base of the shell of L. himactdatus. Towards the 

 centre a number of small rods may be seen to converge. They are 

 cylindrical and bifurcated, and are longest at the edge, where they 

 project around the base of the egg. At the moment when the egg is 

 laid, the two terminal filaments of each small cylinder are soft, and they 

 easily fix themselves to the least asperities of the surface to which they 

 are applied ; they then harden, and the egg is thus firmly attached to the 

 substratum on which the mother has deposited it. The author finds that 

 this fixation apparatus is secreted by the follicle of the egg, the follicle 

 itself being derived from the germinal epithelium. Moreover, the 

 secretion is on the hemisphere of fixation, and this is always the one 

 which is directed outwards. In a perfectly ripe ovary all the eggs have 

 the hemi- ellipsoidal form of the deposited egg, and they are all attached 

 to the wall of the gland by the surface which, after oviposition, will be 

 fixed by means of the fixing-apparatu?. 



* Comptes Rendus, cv. (1887) pp. 87G-8. 



