Turtox News 



Published Monthly by THE GENERAL BIOLOGICAL SUPPLY HOUSE 

 761-763 East 69th Place, Chicago 



Copyright, U. S. A., 1934 



Vol. 12 



APRIL, 1934 



Xo. 4 



NOTES ON COMPARATIVE HISTOLOGY 



Introduction 



The study of Histology is usually con- 

 fined very largely to the examination of 

 the microscopic structure of tissues and 

 organs of the mammalian (including 

 human) body. Comparative Anatomy is 

 confined almost exclusively to the study 

 of the gross structure of vertebrate ani- 

 mals. The comparative study of the 

 microscopic structure of animals is a 

 much neglected field, but one which yields 

 rich returns to both the teacher and the 

 student who will take the trouble to 

 examine microscopically the structure of 

 the various animals which they have dis- 

 sected. The complicated microscopic struc- 

 ture of the human body is more easily 

 understood and its study much more fas- 

 cinating if the simpler beginnings of these 

 complex structures are studied in the 

 lower members of the vertebrate series. 



The organs of the body show an evolu- 

 tionary gradient from the lower to the 

 higher vertebrates in their microscopic 

 structure just as truly as in their gross 

 anatomy. The evolution of the bones of 

 the skull is much emphasized in anatomy 

 courses, but how many students, or even 

 teachers, have seen the excellent evolu- 

 tionary series in the development of com- 

 plexity in the microscopic anatomy of 

 bone? We are told that the teeth of the 

 higher animals have evolved from the 

 placoid scales of elasmobranchs, but few 

 students have had the opportunity of 

 making a direct comparison of sections of 

 placoid scales with sections of human 

 teeth. Lung, skin and blood furnish other 

 interesting examples. 



In this and subsequent articles, we 

 expect to choose certain specific tissues 

 and organs and describe their structure 

 as found in each of the various classes 

 of vertebrates. In most cases we shall 

 begin with Amphioxus, since this animal 

 is considered as typical of primitive pre- 

 vertebrate structure. The text will be 

 almost completely a description of specific 

 structures as seen in slides of the object 



being described. Very little theoretical 

 material will be included. Thus we hope 

 to present a series of descriptions which 

 will be of real value to any teacher who 

 wishes to present some of the comparative 

 aspects of Histology to his students. 

 I. The Skin 



Amphioxus. The epidermis consists of a 

 single layer of low columnar epithelial 

 cells on a dense, hyaline basement mem- 

 brane. It is the toughness of this mem- 

 brane that makes Amphioxus one of the 

 most difficult of animals to section. The 

 dermis is a thin layer of collagenous fi- 

 brous tissue with a few scattered cells, 

 chiefly on its inner surface. This tissue 

 is continuous with the septa between 

 muscle bands. The skin is completely 

 devoid of blood vessels or glands. 



Lamprey. The epidermis is several cells 

 thick, and an excellent example of strati- 

 fied epithelium. The basal cells are colum- 

 nar, those in the middle region more 

 cuboidal, while those at the surface are 

 flattened. However, the very thin 

 squamous cells found on the skins of 

 higher animals are not present. The 

 dermis is a thick layer of white fibrous 

 tissue, with heavy fibres running parallel 

 to the skin surface, and cell bodies well 

 distributed throughout the mass. Numer- 

 ous pigment cells are found on the inner 

 surface of the dermis. Under the skin 

 is a layer of loose fibrous tissue and fat, 

 containing many blood vessels. These 

 latter do not penetrate the skin. In the 

 epidermis, chiefly near its surface, there 

 are numerous small, unicellular serous 

 glands. Deeper in the epidermis are 

 abundant spindle shaped structures which 

 seem to be nerve end organs. Each has 

 one or two nuclei near its surface end, 

 and a central axis which is continuous 

 into the dermis, on which the base of the 

 organ rests. The body of the corpuscle is 

 made up of fine fibres, arranged spirally, 

 the whole suggesting somewhat the ap- 

 pearance of a tactile corpuscle of Meissner 

 from the human skin. 



