34 Growth in length of the Vertebrate Embryo 



or wedge-like strand of thickened proliferating tissue running into 

 the also drawn out but less conspicuous hinder region of the 

 protogenetic embryonal area. 



This proliferating area is what we term the "primitive streak." 

 Along it a groove soon appears. 



Why should it be grooved ? 



Let it be noted that in the first place it is not grooved until 

 it has attained a considerable length. There is no groove at first. 



Secondly the groove disappears long before the streak dis- 

 appears. 



The difference in the characters at these different times is 

 seen in sections, Fig. 17. 



It is at least possible that the grooving may be very largely 

 due to mechanical causes. 



As the deuterogenetic mesoderm passes to either side by being 

 crowded out in the median line, it will tend to be drawn away 

 from the median line by the general expansion of the blastodermic 

 vesicle which is still going on. This must place some tension 

 upon the under surface of the proliferating area, if there is any 

 continuity, so that each side will be drawn away. The result is 

 that a groove arises along the middle of the streak, except at the 

 anterior end where there is less lateral tension, and where there 

 is also an opposing compression in front, due to the antagonistic 

 effects of the protogenetic and deuterogenetic centres. 



Posteriorly also there is no groove or if there were any it would 

 tend to be transverse, as indeed it is actually found in the Sparrow. 



The part of the deuterogenetic proliferating area which will 

 be most expanded on this hypothesis is the latero-anterior, or, 

 speaking morphologically, the dorso-lateral part, that is to say 

 the part equivalent to the dorso-lateral part of the blastopore lip 

 of the Frog, Fig. 18. 



Thus we see that the part which is most expanded is the part 

 which in the Frog gives rise to the neural plate and mesodermic 

 somites. 



If we look at figures of the Rabbit's blastoderm from the time 

 the primitive streak first appears until the time of the formation 

 of the first 7-8 protovertebrae we can see throughout the following 

 features very well defined. 



