Chapter XVI: Some Embryological Methods 121 



in removing the embryo the membranes will be ruptured and the amni- 

 otic and allantoic fluids will escape. 



Submerge the embryo without removing the membranes in a bounti- 

 ful supply of Kleinenberg's picro-sulphuric acid (reagent 25, Appendix 

 B), moving it about gently to rinse off any coagulum that may form on 

 the surface. 



Leave embryos of 6 to 9 mm., 2J hours; 12 to 15 mm., 4 hours; 20 to 

 25 mm., 6 to 8 hours. 



For washing and subsequent treatment see reagent 25, Appendix B. 

 Embryos may be stained in toto in alum -cochineal (reagent 27) or borax- 

 carmine (reagent 32). 



For studying the uterus, placentation (diffuse in the pig), and embry- 

 onic membranes in place, formalin-hardened material may be used after 

 first thoroughly washing it in water. 



For gross dissection of embryos, the specimen should be studied in 

 alcohol under the dissecting microscope. 



Because of the asymmetry of young embryos it is impossible to secure 

 strictly transverse, sagittal, and frontal sections. Minot recommends, 

 therefore, that for practical purposes the plane of section be taken with 

 regard to the head alone irrespective of how it may cut the other parts of 

 the body and suggests the floor of the fourth ventricle of the brain as the 

 guide for orientation. In his Laboratory Text-Book of Embryology he 

 especially recommends that each student prepare sections of the follow- 

 ing stages of pig embryos: 9 mm., transverse and sagittal, frontal of the 

 head; 6 mm., transverse, frontal of the head; 17 mm., transverse and 

 sagittal, frontal of the head; 20 mm., transverse and sagittal, frontal of 

 the head; 24 mm., frontal of the head. 



8. For the Stages of Maturation, Fertilization, and Segmentation in Mam- 

 mals white mice will prove most useful because these processes are better 

 known in them than in other mammals; furthermore, an abundance of 

 material may be procured. The ovum, however, is extremely small, 

 measuring only 59 microns in diameter. It is surrounded by a very thin 

 zona pellucida (1.2 microns). In the majority of ova (80 to 90 per cent.) 

 only a single polar body is formed; it corresponds in every way appar- 

 ently to the second polar body when two are formed. It has been 

 inferred, therefore, that probably in the mouse the first polar body ordi- 

 narily forms at an early stage in the history of the germ cells and is thus 

 overlooked by observers. 



When two polar bodies have been observed, the first always appears 

 while the egg is yet in the Graafian follicle of the ovary; the second, after 

 the egg has entered the Fallopian tube. When only one polar body is 

 present it, likewise, appears after the egg has entered the tube. The 



