396 AN INTRODUCTION TO ZOOLOGY 



segments or somites. At the time of closure of the neural tube, 

 three or four pairs of somites have been formed ; the first of them 

 lies at the end of the fore gut, and thence the process of somite 

 formation passes posteriorly. It is noteworthy that such segmenta- 

 tion does not extend forward to the head region or laterally to the 

 lateral plates. Here, then, we have the embryo in approximately 

 the same stage of development as was reached in Amphioxus 



Gallus, the Fowl. 



In the fowl's egg we have an example of the extreme 

 telocithal condition, for there is a very large amount of yolk all 

 massed at one pole. This is a matter of considerable interest from 

 the comparative point of view, since it leads to certain modifications 

 in the process of development resulting, among other things, in the 

 production of certain membranes. The mammal, whose egg is small 

 and almost yolkless, has been derived from an ancestral form with 

 a heavily yolked egg, and consequently we find certain peculiarities 

 in its development paralleled in the fowl's egg, and the membranes 

 are put to quite other uses, as we shall see. Fertilisation is internal 

 and effected at the top of the oviduct, and segmentation commences 

 straight away, so that by the time it is laid the process is well under 

 way. 



At oyulation the egg consists only of the so-called yolk which 

 passes to the oviduct (in the fowl only the left ovary and oviduct are 

 functional), where it has added to it the tertiary egg membranes. 

 The ovum leaving the ovary is surrounded by a thin vitelline mem- 

 brane, and in spite of its large size is only a single cell whose nucleus 

 and active cytoplasm is confined to a small whitish area about 

 3 mm. in diameter on one side of it. The germinal disc, as this 

 spot is called, is situated upon and continuous with the yolk, and the 

 latter is composed of two different varieties of material, one white 

 and the other yellow. A layer of the white yolk underlies the 

 germinal disc and a neck of it extends downwards into the centre 

 of the yolk, where it swells out to form a flask-shaped mass, the 

 latebra. Around this the yolk is arranged in concentric layers of 

 alternate thick layers of yellow and thin layers of white. The 

 cytoplasm around the disc where it joins the white yolk is slightly 

 darker in colour and forms a ring known as the periblast. When the 

 egg is laid we find that there has been added to the ovum a thin 

 layer of very dense albumen, the chalaziferous layer, which is 

 continued out at each side into a tough twisted cord, the chalaza, 

 whose function it is to keep the germinal disc uppermost, however 

 much the egg may be turned about. Outside this is a thick layer of 



