DEVELOPMENT OF BIEDS AND MAMMALS 235 



Extra sperm. 



Pol. bod. 

 Pronuc. \ Seg. nuc. 



the two layers of which are separated by the air space at the 

 broad end, and a limy secretion gives them a final covering 

 of a calcareous shell which in most birds receives besides a 

 deposition of pigment. The sperm has the usual flagellate 

 structure, and is provided with a long head terminating in a 

 sharp process. Fertilisation takes place in the upper part of 

 the oviduct. As in Elasmobranchs, several sperms enter 

 the egg, but fertilisation 

 usually is effected by one, 

 the rest during develop- 

 ment being kept outside 

 of the area of segmen- 

 tation. 



Blastoderm. S egmen- 

 tation takes place in the 

 oviduct and results in 

 v the formation of a white 

 patch of cells at the 

 animal pole, the blasto- 

 derm, known also as 

 the cicatricula, or tread. 

 During the process of 

 segmentation in the ovi- 

 duct a superficial layer of 

 cells is developed which 

 yields by horizontal 

 divisions cells under- 

 neath, and the blasto- 

 derm reaches a thickness 



of several layers. This is resolved into a layer of ectoderm 

 in the form of a single layer of cylindrical cells, and an 

 endoderm which is being reduced to a single layer in the 

 middle of the blastodermal area, but still is many-layered 

 at the margin. Between the two a segmentation cavity is 

 indicated. It is in this condition usually that the egg is laid, 

 and in this phase it may remain for a long period in a suspended 

 state of development, and it does not undergo further develop- 

 mental changes until it is placed in a temperature approaching 

 that of the bird (fig. 120). 



Endoderm 



Ectoderm 



FIG. 120. Segmentation. The upper four 

 figures refer to the pigeon, and are 

 after Blount ; the last figure to the 

 fowl, and is after Duval. The stages 

 represented take place in the oviduct. 



