246 MARY BLOUNT. 



gest the spreading of the blastoderm over the unsegmented part 

 by cells organized around the superficial periblast nuclei. The 

 blastoderm instead of having an almost perpendicular margin as 

 in Fig. 10, comes to lie over the periblast. These figures show 

 other additions of cells besides those at the extreme margin, 



8. The Germ Wall. — The term Keimwall was first used by 

 His in 1866. In his description of the germ wall of the hen's 

 egg, His (5) says that in the first hours of incubation the white 

 yolk on which the border of the germ area rests is grown 

 through with cells of the germ, and it forms a peculiar structure 

 with protoplasmic frame work enclosing white yolk spheres. 

 To this structure, His gave the name " Keimwallgewebe" or or- 

 ganisirten Keimwall. 



His (5) also says that he was able to follow " wie tiefliegende 

 Zellen des Keimes vermoge ihre sehr ausgesprochenen amoboi- 

 den Beweglichkeit die ihnen benachbarten Dotterkorner und 

 Dotterkugeln in sich aufnehmen." 



In another paper His (4) says, " Wahrend der ersten Zeit der 

 Bebriitung entsendet die untere Schicht jenes Randtheils Fort- 

 satze zwischen die Elemente des Keimwalles, so dass diese gros- 

 sentheiles in ein Geriist archiblastischen Protoplasmas einge- 

 schlossen werden." 



This conception of His is based upon a study of hen's eggs 

 during the first few hours of incubation. He makes no reference 

 to the germ wall in the unlaid egg. A study of the pigeon's 



Fig. 14. Margin of transverse section of a pigeon's egg about 28 hours after fer- 

 tilization. The periblast nuclei, except the most peripheral one, were all in this sec- 

 tion. 



egg in close stages of development before laying gives quite a 

 different conception of the germwall, — particularly as to the 

 origin of the nuclei. They are periblast nuclei, and are not de- 

 rived from the " tiefliegende Zellen des Keimes." Such nuclei 

 are shown in Fig. 16 which represents the margin of the blasto- 

 derm in transverse section of a pigeon egg six hours before lay- 

 ing. It is, of course, a younger stage of the germ wall than is 



