GERM LAYERS. 97 



Transverse sections cut after the gastrula has begun to elongate to form the embryonic 

 body, prepared by the same technic as above, will show the mesoderm at each side of the 

 primitive gut. 



Surface views are very instructive in cases of discoidal cleavage. The primitive streak 

 is well shown in the hen's egg during the second half of the first day of incubation. The 

 blastoderm is removed from the surface of the yolk, fixed in Zenker's fluid, stained in toto in 

 borax-carmin and mounted in toto in xylol-damar (see Appendix) . 



A blastoderm of the same stage (second half of first day), fixed in Zenker's fluid or in 

 Flemming's fluid, preferably the latter, cut in paraffin and stained with Heidenhain's hema- 

 toxylin, will show instructive pictures of the three germ layers before the splitting of the 

 mesoderm; and will also show the fusion of all three germ layers in the line of the primitive 

 streak. 



Surface views of the chick blastoderm, during the second day of incubation, are also 

 instructive as to general topography, showing the neural groove (or tube if late in the second 

 day) and its relation to the primitive streak, the primitive segments, the area pellucida and 

 the area opaca. The blastoderm is removed from the surface of the yolk, fixed in 

 Zenker's fluid, stained in toto in borax-carmin, and mounted in toto in xylol-damar (see 

 Appendix). 



Sections of a blastoderm of the same stage (second day) are especially valuable in showing 

 the early conditions of the germ layers after the primitive segments have appeared and the 

 mesoderm has split into two layers. The blastoderm is fixed in Flemming's or Zenker's 

 fluid, cut transversely in paraffin and stained with Heidenhain'i hematoxylin. The ectoderm 

 and neural groove (or tube), the mesoderm (primitive segments, parietal and visceral layers 

 with enclosed ccelom), and the entoderm and notochord are very clearly shown. The visceral 

 mesoderm usually contains a number of developing blood vessels. 



For methods of procuring mammalian blastodermic vesicles and preparing them for 

 study, see page 630. 



References for Further Study. 



Assheton, R. : The Reinvestigation into the Early Stages of the Development of the 

 Rabbit. Quart. Jour, of Mic. Sci., Vol. XXXVII, 1894. 



Assheton, R. : The Segmentation of the Ovum of the Sheep, with Observations on the 

 Hypothesis of a Hypoblastic Origin for the Trophoblast. Quart. Jour. 0} Mic. Sci., Vol. 

 XLI, 1898. 



Van Beneden, E.: Recherches sur les premiers stades du developpement du Murin 

 (Vespertilio murinus). Anat. Anz., Bd. XVI, 1899. 



Bonnet, R.: Lehrbuch der Entwicklungsgeschichte. Berlin, 1907. 



Bonnet, R.. Beitrage zur Embryologie der Wiederkauer gewonnen aus Schafei. Arch. 

 }. Anat. u. Physiol., Anat. Abth., 1884, 1889. 



Bonnet, R.: Beitrage zur Embryologie des Hundes. Anat. Hejte, Bd. IX, 1897; Bd. 

 XVI, 1901. 



Bryce, T. H.,and Teacher, J. H.: Early Development and Imbedding of the Human 

 Ovum. Glasgow, 1908. 



Harper, E. H. : The Fertilization and Early Development of the Pigeon's Egg. Am. 

 Jour. 0} Anat., Vol. Ill, 1904. 



Hatschek, B.: Studien iiber Entwicklung des Amphioxus. Arbeiten aus dem zool. 

 Instit. zu Wien, Bd. IV, 1881. 



