118 



Methods. 

 The methods for preparing and securing this material have already 

 been fully described^), so that the following brief account of the fixing 

 and staining methods will suffice. The entire yolk is fixed for one 

 hour in a mixture of 92 parts of Kleinenberg's strong picro-sulphuric 

 plus eight parts of glacial acetic. The egg is washed in 70 per cent 

 alcohol until entirely free from the picric acid, and placed in 80 per 

 cent. At this point a carefully oriented block of yolk containing the 

 blastoderm is cut out. This block is carried through the higher alco- 

 hols, cleared in cedar oil, and embedded in paraffin. The sections 

 were cut ten microns thick and stained on the slide with Delafield's 

 haematoxylin. Sections prepared according to the above method give 

 excellent nuclear figures, with little or no distortion of the nuclear 

 membrane. 



Description of Cells and Types of Division. 



The cells of the primary ectoderm and entodern are heavily laden 

 with yolk spherules, but in the neighborhood of the nucleus the spherules 

 are usually wanting — this region being either clear or occupied by 

 small granules only (Figs. 13—16). The nucleus is surrounded by a 

 distinct membrane, which makes it especially favorable for the study 

 of amitosis. Within the nucleus is one and sometimes two nucleolei 

 (karyosome — Figs. 8, 10). The rest of the nuclear substance has a 

 reticular structure, in which chromatin granules are often embedded 

 (Figs. 2, 17, 18). 



Two types of amitotic division occur in the pigeon's egg. In most 

 cases the nucleus elongates in the direction transverse to the plane 

 of the future division, after which a constriction in the nuclear mem- 

 brane appears about the entire circumference of the nucleus (Figs. 1 c, 

 2 b, 3, 4). This constriction continues to deepen until a complete 

 division is effected (Figs. 1 a, 6, 7). A modified form of this type is 

 found in those cases in which the constriction proceeds from one side 

 (Figs. 9, 11, 12). In such cases the nucleus usually does not elongate 

 previous to the appearance of the constriction. In other cases a 

 nuclear plate is laid down across the nucleus, and division consists in 

 a splitting of this plate (Figs. 2 a, 3, 5, 7). 



Whether in all cases the division of the nucleolus precedes that 

 of the nucleus in the above types is difficult to determine. In most 



1) J. Thus. Patterson, "On Gastrulation and the Origion of the 

 Primitive Streak in the Pigeon's Egg". Biol. Bull., Vol. 13, '07, No. 5. 



