68 SUMMARY OF CURRENT RESEARCHES RELATING TO 



on the anterior and superior face of the cerebral ganglia, and the presence 

 of two always symmetrical ganglia at the angle made between the 

 CBSophagus and the lingual pouch are constant phenomena ; these two 

 ganglia form the stomatogastric centre, and give off a definite system of 

 nerves. When the masticatory apparatus is robust and is moved by 

 powerful muscles a series of small ganglionic centres make their 

 appearance. The author describes the different arrangements which obtain 

 in various gastropods, in which parts of the digestive tract are specially 

 modified, and he points out that the corresponding modifications in the 

 nervous supply must not be thought to in any way modify the interpreta- 

 tion of the stomatogastric centre, which always remains the same in its 

 central part, and is only modified in a part of its periphery to respond 

 to new wants. 



Development of Genital Apparatus of Stylommatophorous Pul- 

 monata.* — Dr. J. Brock finds the first rudiment of the generative organs 

 of Stylommatophorous Pulmonata in larvse just previous to extrusion ; at 

 the side of the right central ganglion, in a shallow depression, there lay 

 directly beneath the cutis a fine cord of cells with a distinct lumen; 

 posteriorly, the duct took a somewhat upward direction, and lay by the 

 outer lower angle of the central ganglia. The wall of this primary genital 

 duct consists of a layer of radially arranged cubical cells. It is to be 

 noted that it is to be found on the right side only. This appears to be the 

 structure which Kouzeaud calls the " bourgeon primitif," but Brock knows 

 of no fact which would justify an ascription to it of an ectodermal origin. 



The next stage studied was found in free forms, about 2 mm. long ; the 

 primary duct is longer, and the hermajihrodite gland is beginning to be 

 formed ; the first rudiment of the penis is found in a spindle-shaped 

 outgrowth, while a thickening of the median wall is the earliest indication 

 of the dart-sac ; the hermaphrodite duct is also beginning to be formed. 

 The author thinks the evidence that in the Pulmonata, at any rate, the 

 whole of the generative apparatus is derived from the mesoblast, is 

 complete. 



In somewhat older animals the genital duct and the hermaphrodite 

 gland become connected by the hermaphrodite duct, the whole of this 

 appearing at almost the same time ; it is important to note that in the 

 Pulmonata the germ-gland and the efferent duct were primitively separate. 

 The other changes that occur in this stage are increase in length of the 

 duct and of the rudimentary penis ; the outgrowing of the penis results in 

 the appearance of the distally placed genital atrium. In specimens hardly 

 any older the penial swelling becomes constricted off from the primary 

 genital duct as a blind sac, and the hermaphrodite gland begias to be 

 broken up into lobules. The constriction of the penis goes on rapidly, 

 and the organ becomes greatly increased in size. 



In animals 4-5 mm. long three important changes take place ; these 

 are the development of the vas deferens as an outgrowth of the penial blind 

 sac, the division of the primary genital duct into a male and a female duct, 

 and the breaking through of the outer genital orifice ; there is no evidence 

 at all to support the supposition that an ectodermal invagination takes 

 any part in the formation of this orifice. The penis is now completely 

 constricted off from the primary genital cord, and the atrium also becomes 

 distinct. The hermaphrodite gland becomes more lobulated, and elements 

 are developed in it which may be regarded as primitive ova ; these are 

 large rounded cells with a large round nucleus and a large nucleolus. 



* Zcitscbr. f. Wiss. Zool., xliv. (1886) pp. 333-95 (4 pis.)- 



