CHAPTER XXV. 259 



Anura, Teleostea, Cyclostomata, Echinodermata, and many Vermes 

 and Coelenterata. 



In the case of the Amphibia, both the female and the male should 

 be laid open, and the ova should be extracted from the uterus 

 and placed in a watch-glass or dissecting dish, and treated with 

 water in which the testes, or, better, the vasa differentia, of the 

 male have been teased. 



Females of Teleostea are easily spawned by manipulating the 

 belly with a gentle pressure ; and the milt may be obtained from 

 the males in the same way. (It may occasionally be necessary, 

 as in the case of the Stickleback, to kill the male, and dissect out 

 the testes and tease them.) The spermatozoa of fish, especially 

 those of the Salmonidse, lose their vitality very rapidly in water ; 

 it is, therefore, advisable to add the milt immediately to the spawned 

 ova, then add a little water, and after a few minutes put the whole 

 into a suitable hatching apparatus with running water. 



Artificial fecundation of Invertebrates is easily performed in a 

 similar way. For methods of artificial Parthenogenesis see HARVEY, 

 Biol Bull Wood's Hole, 1910, p. 269. 



579. Superficial Examination. The davelopment of some animals, 

 particularly some invertebrates, may be to a certain extent followed 

 by observations of the living ova under the microscope. This may 

 usefully be done in the case of various Teleosteans, such as the 

 Stickleback, the Perch, Macropodus, and several pelagic forms, and 

 with Chironomus, Asellus aquaticus, Ascidians, Planorbis, many 

 Coelenterata, etc. 



Some ova of insecta and Arachnida which are completely opaque 

 under normal conditions become transparent if they are placed in 

 a drop of oil ; if care be taken to let their surface be simply impreg- 

 nated with the oil, the normal course of development is not interfered 

 with (BALBIANI). 



580. Fixation.* Osmic acid, employed either alone or in com- 

 bination with other reagents, is an excellent fixing agent for small 

 embryos, but not at all a good one for large ones. It causes cellular 

 elements to shrink somewhat, and therefore brings out very clearly 

 the slits that separate germinal layers, and any channels or other 

 cavities that may be in course of formation. 



In virtue of its property of blackening fatty matters, myelin 

 amongst them, it is of service in the study of the development of 

 the nervous system. 



Chromic acid is indispensable for the study of the external forms 



172 



