4 Growth in length of the Vertebrate Embryo 



As development proceeds, the blastoderm in each case becomes 

 expanded and it gradually surrounds the whole yolk; but long 

 before it does so there appears a thickening of the outer layer just 

 inside the blastopore rim, and the rim itself also is thickened 

 about this region. The inner edge of this thickened knob is the 

 anterior end of the future neural tube, and the rim is therefore 

 the most posterior part of the embryo. As the blastoderm 

 expands this thickening is apparently extended inwards from the 

 rim. The process that is really taking place is a growth of the 

 thickened area which is usually called the "embryo" extending 

 backwards and keeping pace with the expanding blastopore rim. 

 Accordingly this rim is often called the germ ring. 



In the Teleostean, which gives a much truer idea of the growth 

 processes than does the Elasmo- 

 branch, the germ ring soon reaches 

 the equator of the egg. By this 

 time the "embryo" has also 

 . increased a corresponding amount 

 ' in length, and its anterior end is 



definitely differentiated into the 

 anterior portion of the fish, Fig. 2. 

 After the equator has been 

 attained the germ ring quickly 



Fig. 2. Diagram to show the pro- spreads on to the lower pole and 

 gress of the germ nng as it travels . , . . . , 



over the yolk at four successive m a short time the whole egg IS 



8ta s es - enveloped. 



b blastopore ; gr germ ring ; mi < i 



l, 2, 3, 4, successive stage! The part of the germ ring 



opposite to the part where the 



thickening occurs, travels much more quickly over the yolk than 

 the thickened part. 



Thus it is seen that the blastopore becomes more and more 

 reduced until it actually closes. By this time the greater part 

 of the "embryo" has been laid down, in fact all of it except the 

 tail; and we have watched the growth in length of the embryo 

 from the time when the head region first appeared until just 

 before the tail begins to grow. The question is how has this 

 growth taken place? 



When once an embryo is formed it may increase either in 



