A very close look at a dogfish.

Feeling somewhat under the weather I have spent the day indoors peering down my microscope and taking photographs of some old histological slides.
This is one that I prepared earlier, in February 1964. It is a cross section of the the testis of a dogfish, stained, if I remember correctly, with iron haematoxylin. 
As in mammals, the main roles of the dogfish testis are to produce spermatozoa and steroid male hormones, but there are notable differences in their structure. Whereas in mammals the sperm are produced in long seminiferous tubules in the dogfish, and other sharks, they are produced in spermatocysts which are discrete membrane-bound spherical units, rather like a bunch of microscopic grapes. In the photograph you can see sections through a number of these units each filled with sperm at different stages of development.