2C4 PROGRESS OF MICROSCOPICAL SCIENCE. 



been done years before by a French zoologist ; but there are some 

 points of novelty in his pai)cr. Particularly interesting is his 

 description of the penis of this animal. Looking down, he says, 

 upon the extremity of the extruded sheath, when expanded, a minute 

 orifice may be observed through which the penis is projected. It is 

 a dark-coloured, wire-like organ — presumably chitinous — which is 

 capable of jirotrusion for about half the length of its sheath ; but of 

 its structure he is able to give no further description ; in fact, he has 

 only seen the extrusion of the organ in some four or five instances 

 out of the dozens of male fleas he has had under observation, and on 

 thes3 occasions it was protruded and retracted with such rapidity that 

 the eyes could hardly follow it. He has no doubt, however, that it 

 constitutes one of the coiled rods or chitinous fibres, and that it is 

 projected by the uncoiling, and withdrawn by the release of the 

 spring-like coil. With regard to the copulation of this insect he 

 says that when young and transparent specimens of the insect are 

 selected this important process can be seen with remarkable clearness, 

 and the animals being so closely locked together may be manipulated 

 with great facility. Even when placed between the glasses of the 

 compressor they will endure an amount of comj)ression quite suffi- 

 cient to render the abdomen of each insect jjerfectly transj)arent 

 without the interruption of the copula. "When thus examined the 

 extremity of the sheath of the penis may be seen to be continually 

 opening out and closing up, and by this action the spinous processes 

 attached to the extremity of the sheath appear, as it w^ere, to grasp 

 the ovarian clusters which he has described ; but he has not been 

 able to observe the jjrotrusion of the penis itself during the copula, 

 nor at any time to distinguish sj)ermatozoa in the female. 



Description of certain LejndodemJroid Stems. — Tlie following ac- 

 count of certain stems recently examined by Professor Williamson has 

 been sent by him to the Eoyal Society, and is jiublished in the ' Proceed- 

 ings of the Pioyal Society,' No. 133. The examples vary from the very 

 youngest, half-develoj)ed twigs, not more than yVth of an inch in 

 diameter, to arborescent stems having a circumference of from two to 

 three feet. The youngest twigs are composed of ordinary parenchyma, 

 and the imperfectly-developed leaves which clothe them externally 

 have the same structure. In the interior of the twig there is a single 

 bundle, consisting of a limited number of barred vessels. In the 

 centre of the bundle there can always be detected a small amount of 

 primitive cellular tissue, which is a rudimentary pith. As the twig 

 expanded into a branch, this central pith enlarged by multii^lication 

 of its cells, and the vascular bundle in like manner increased in size 

 through a corresj)onding increase in the number of its vessels. The 

 latter structure thus became converted into the vascular cylinder, so 

 common amongst Lepidodendroid plants, in transverse sections of 

 which the vessels do not appear arranged in radiating series. Simul- 

 taneously with these changes the thick parenchymatous outer layer 

 becomes differentiated. At first but two layers can be distinguished — 

 a thin inner one, in which the cells have square ends, and are disposed 

 in irregular vertical columns, and a thicker outer one consisting of 



