ORIGIN OF THIS SEXUAri CELIiS IN HYDBOIDS. 619 



The cells of the outer layers which have pritnitively an 

 irregular polygonal form become elongated and placed perpen- 

 dicularly to the plane of the basal lamella, at the same time 

 changing in character, losing their granules and becoming 

 clear. Such a modification takes place just above a group of 

 sexual cells. 



The modified ectodermal cells now form a rounded tubercle 

 which becomes nipped at the base by a circular groove and 

 will now penetrate the perisarc. The perisarc in fact is 

 gradually eaten away by a chemical action. That there is no 

 mechanical pushing and gradual thinning out of its substance 

 can be seen by tracing the parallel striae which stop short at 

 the edges of the window which is formed. This eating away 

 of the perisarc is a most remai'kable process. The perisarc is 

 a chitinous substance and only dissolves with the greatest 

 difficulty in concentrated acids or alkalies. 



Experimenting upon Plumularia echinulata, Weiss- 

 niann found that the perisarc entirely resists the action of 

 sulphuric and hydrochloric acids for five days, as well as that 

 of caustic potash : it did, however, dissolve in the latter at the 

 end of a month. 



Numerous organic bodies resist the action of strong acids or 

 alkalies while they are attacked by weaker solutions, Weiss- 

 mann has therefore tried all stages of dilution. In a '1 per 

 cent, solution of potash the perisarc had not, however, com- 

 pletely dissolved at the end of a fortnight. 



There is a further curious point ; even while these cells are 

 dissolving the outer layers of perisarc they do not attack the 

 youngest (innermost) layer which they push before them. This 

 young layer, which Weissmaun terms the cambium layer, 

 present differences both chemical and physical from the older 

 layers, it stains more strongly and more readily with carmine,! 

 and, moreover, is soft and elastic, as may be seen during the 



^ I have observed a similar difference in tiie cliitinous layers of the lens of 

 the central eye of a young Limulus, one portion altlHuigh here occupying 

 not an internal but a central position, stains deeply when treated with borax- 

 carmine while the remainder remains unstained. 



