74 NOTES AND MEMOEANDA. 



doubts which must certainly now be considered comj)letely disposed 

 of after the proof of the separation of the sexes in Halisarca and in 

 Aplysilla sulfurea, where, according to F. E. Schulze's investigations, 

 male and female elements occur. Further observations may serve to 

 give greater weight to Schulze's statement. 



Dr. Keller endeavoured last year to examine the facts in question 

 in Spongilla. A separation of the sexes seems to occur in this fresh- 

 water sponge also — at least, he found all through the summer small 

 specimens which contained neither eggs nor larvae, but, on the con- 

 trary, were closely filled, especially in the spring, with sperm-follicles 

 in the most varied stages of develoj)ment. The smaller specimens were 

 attached to the cases of the larvae of Phrt/ganea, and in these were found 

 almost invariably sperm-balls. It is just these which must be espe- 

 cially adapted for fecundation. 



The spermatic elements are enclosed in a special receptacle, and 

 when mature move about in it with great activity. 



Each follicle is surrounded by numerous cells (nutritive migratory 

 cells). If a mature follicle bursts, or if it is made to burst by pressure 

 on the covering glass, the sperm-cells disperse, and move about in 

 large numbers (their heads disposed towards each other) for hours at 

 a time with great briskness. 



In the younger follicles the movement is wanting ; the contents 

 are numerous closely-pressed round elements. It is to be assumed 

 in the case of Spongilla, therefore, that the sperm-follicle with its 

 contents originates from a single cell by continual division. 



These are recommended as a desirable object for demonstration in 

 a course of zootomy, as the movements in the follicles last a con- 

 siderable time. The small Spongilla found attached to the cases of 

 Phryganea in May and June are the best adapted for this purpose. 



The exact Orientation of the principal Section of Nicols in 

 Polarizing Apparatus. — It is sometimes necessary to be able tj de- 

 termine the orientation of the lirincij^al section of the polarizers and 

 analyzers — Nicols, double refracting prisms, &c. 



It may be done simply and with precision by illuminating the 

 apparatus, in order to adjust it, with yellow light, and interposing 

 a diaphragm between the polarizer and analyzer, oue-half of it being 

 covered with a half undulation plate of thin quartz parallel to the 



axis. 



This diaphragm can always be put in position. Polarizing appa- 

 ratus generally have either a system of lenses or a single lens, which 

 can then be used to view the diaphragm. If this is not the case, a 

 small auxiliary lens can be used. The interior margin of the plate 

 separates the two half-disks, and produces a well-defined line. 



Let us suppose that it is required to fix a Nicol so that its prin- 

 cipal section shall make a given angle with certain reticular threads, 

 &c. The problem is reduced to placing the margin of the plate in the 

 desired position, and, as this is a well-defined line, the optical and 

 mechanical means are not wanting. 



The Nicols are then adjusted with respect to the plate. To do 

 this, the polarizer is placed so that its section is approximately in the 



