122 Animal Microloyy 



spermatozoon reaches the egg in the upper end of the Fallopian tube 

 some time after coitus. Formation of the single (or the second) polar 

 body and the entrance of the spermatozoon may be found in the same egg. 



The female comes into heat 21 days after littering and coitus can 

 take place only when she is in heat because at other times the vagina is 

 closed. During heat the periovarial space and the beginning of the Fal- 

 lopian tube are distended with a clear fluid. Ovulation occurs before or 

 at the time of coitus. (See " Die Befruchtung und Furchung des Eies 

 der Maus" by Sobotta: Archiv fur mikroskopische Anatomie, Vol. XLV 

 [1895], pp. 15-93, Plates II-IV.) 



9. Artificial Fecundation when it can be practiced is the most convenient 

 means of securing early stages of development. This is possible with 

 many worms, coelenterates, echinoderms, cyclostomes, teleosts, and anu- 

 ran amphibia. 



a) In echinoderms (e. g., sea urchin) the female is cut open and a num- 

 ber of the living eggs transferred to a watch-glass which contains fresh 

 sea water. The testes of a male are teased out in sea water and a drop 

 of the mixture is conveyed by means of a pipette into the dish containing 

 eggs. Immediately upon fertilization a membrane forms around each 

 fertilized egg. In about 40 to 50 minutes after fertilization the signs of 

 the first cleavage should appear. The blastula forms in about 6 hours, 

 and the gastrula in about 12 hours. For the study of fertilization, etc., 

 the following stages should be fixed in picro-acetic (reagent 23, Appen- 

 dix B) for 30 minutes and stained in Conklin's hematoxyliu (reagent 48); 

 5 minutes after fertilization, nucleus giving off polar bodies; 30 minutes 

 after fertilization, approaching pronuclei; 50 to 55 minutes after fertiliza- 

 tion, division of nucleus (mitotic figure) in the first cleavage. 



b) In amphibia (e. g., frog) both male and female are cut open, the 

 vasa deferentia or testes are teased out in a watch-glass full of water and 

 the ova are then removed from the lower ends of the oviducts and placed 

 in this water. After fertilization the eggs should be placed in glass 

 dishes in not over 4 inches of water. Many eggs should not be placed 

 in one dish. See also memorandum 5. 



c) In teleosts the eggs are obtained by stripping the female when she 

 is in spawning condition. At stich times the eggs are loose in the body 

 cavity and may be pressed out by gently manipulating the belly of the 

 fish. The head of the fish should be held in one hand, the tail in the 

 other and the thumb or the thumb and forefinger used to press out the 

 ova; the vent of the fish should remain submerged in water. The milt 

 of the male is obtained in flu 1 ; same manner and in the same dish. Eggs 

 and sperm are then gently stirred about by means of a feather to insure 

 thorough mixing. However, in some teleosts (e. g., stickleback) it is 

 necessary to kill the male and tease out the testes. In the dinner (Cten- 



