44 Growth in length of the Vertebrate Embryo 



anterior part of the blastoderm. These are the first traces of the 

 middle layer of cells or mesoderm. 



The ectoderm even at the time of laying is thicker near the 

 centre of the area pellucida than elsewhere. In Gallus after some 

 5-10 hours of incubation this thickening is more marked still over 

 a wide area occupying the more anterior part of the area pellucida, 

 and a little later still a much more decided thickening can be 

 seen in the more posterior part of the area pellucida extending 

 from about the centre of the area backwards nearly to the margin 

 of the posterior part of the area opaca. This thickening is the 

 first sign of a structure which has long had "the name of primitive 

 streak, but it should be recognised at once as the growing point 

 for the increase of length of the embryo, while the less well-defined 

 thickening of the ectoderm in front is the beginning of the plate 

 of tissue which will ultimately form the anterior part of the 

 central nervous system the fore and perhaps mid brains. 



Such is the condition at the llth hour of incubation, Fig. 25. 

 Transverse sections through these two regions reveal certain well- 

 marked differences. A transverse section through the anterior 

 region along A-B shows us a decided thickening of the ectoderm, 

 but it is a wide diffuse thickening, having a quite sharp inner 

 surface; a few scattered cells are present between the layers 

 which are the mesoderm cells already referred to : while a section 

 along C-D shows a much more restricted area of thickening 

 which has a ragged inner surface from which mesoderm cells 

 are being actively proliferated. 



Up till now in surface view the whole blastoderm has appeared 

 circular, as also the area opaca and the area pellucida. But now 

 the outline of the area pellucida becomes elongated and it can be 

 clearly seen that this elongation of the area pellucida is an 

 elongation backwards, and that this elongation involves the 

 elongation of the primitive streak. By the 18th hour the primitive 

 streak has stretched backwards and it now reaches from the 

 centre of the original circular area almost to the area opaca. It 

 is a more or less straight line, and is deeply grooved along its 

 whole length except at the two ends. The groove is deepest at 

 the anterior end, and it ceases abruptly against a rather well- 

 marked mass of cells which is sometimes called "Hensen's knot" 



