30 DEVELOPMENT AND GROWTH OF 



Keimblatter im Hiihnerei," Sitzungsberichte der K.Akademie der 



Wissenschaften in Wicti, 1868. Oellachcr, " Untersuchung iiber 



die Furchung und Blatterbildung im Hiihnerei," Studien aus dem 



Institut fur Experim. Pathologiein Wien, 1870 (pp.54 -74), and 



Dr Klein, Ixiii. Bande der Sitz. dcr K. Acadamie der Wiss. in 



Wien, 1871. 



The unincubated blastoderm (PI. I, fig. i) consists of two 

 layers. The upper layer is composed of a single row of columnar 

 cells. Occasionally, however, the layer may be two cells thick. 

 Th- cells are filled with highly refracting spherules of a very 

 small size, and similar in appearance to the finest white yolk 

 spherules, and each cell also contains a distinct oval nucleus. 

 This membrane rests with its extreme edge on the white yolk, 

 its central portion covering in the segmentation cavity. From 

 the very first it is a distinct coherent membrane, and exhibits 

 with silver nitrate a beautiful hexagonal mosaic of the outlines 

 (PI. I. fig. 6) of the cells. The diameter of the cells when 

 viewed from above is from ^oVo swo f an mcn - The under 

 layer is very different from this : it is composed of cells which 

 are slightly, if at all, united, and which vary in size and appear- 

 ance, and in which a nucleus can rarely be seen. The cells 

 of which it is composed fill up irregularly the segmentation 

 cavity, though a distinct space is even at this time occasionally 

 to be found at the bottom of it. Later, when the blastoderm 

 has spread and the white yolk floor has been used as food, 

 a considerable space filled with fluid may generally be found. 



The shape of the floor of the cavity varies considerably, 

 but it is usually raised in the middle and depressed near the 

 circumference. In this case the under layer is perhaps only 

 two cells deep at the centre and three or four cells deep near 

 the circumference. 



The cells of which this layer is composed vary a good deal 

 in size ; the larger cells being, however, more numerous in 

 the lower layers. In addition, there are usually a few very large 

 cells quite at the bottom of the cavity, occasionally separated 

 from the other cells by fluid. They were called formative cells 

 (Bildungselemente) by Peremeschko (loc. cit.) ; and, according 

 to Oellacher's observations (loc. cit.), some of them, at any rate, 

 fall to the bottom of the segmentation cavity during the later 



