28 Growth in length of the Vertebrate Embryo 



There is one other point to be insisted upon in considering 

 this question of growth in length in Amniota not only is there 

 this fluid enclosed in the archenteron, but there is an increasing 

 amount of 'pressure, so that we have here to deal not only with 

 a hydrostatical condition but also with a hydrodynamical con- 

 dition, and this must have its effect upon the mechanics of 

 development. 



The development of the Rabbit. 



We will now take the formation of growth centres in the 

 embryo of the Rabbit in more detail. This animal is chosen 

 because of all vertebrates none other has been so far described 

 that shows in so distinct a manner the relations of the primary 

 and secondary growth centres. 



We know that the blastocyst of the Rabbit attains a large size 

 from causes within itself before it becomes influenced by the 

 walls of the uterus, for this reason it is more suitable than are the 

 blastocysts of most mammals, and because of the absence of yolk 

 it is more suitable than those of birds or reptiles, for study of 

 growth centres. 



In the accompanying Fig. 15 the early stages in an ordinary 

 Eutherian are shown such as that of the Pig, which we may take 

 as typical of the sub-class in which there is some, but very little, 

 indication of what has been called inversion (entypy). 



The chief points are that the egg has but little yolk though 

 some is present in the form of fat drops. The segmentation is 

 holoblastic, the four segment stage is criss-cross, Fig. 15 A, and 

 the result of segmentation is a solid sphere of cells all much 

 of the same size this stage is termed the morula, Fig. 15 B. 

 A cavity arises by the formation of vacuoles within some of the 

 more internal cells, Fig. 15 C. The vacuoles run together, the 

 fluid increases and the cavity gets larger and larger till it occupies 

 a position that leaves only a heap of cells at one pole, and a layer 

 reduced to a single cell in thickness elsewhere, Fig. 15 D. 



The heap of cells becomes flattened, and three layers are 

 distinguishable at this point ; the outer is termed trophoblast, the 

 middle ectoderm, the inner endoderm, Fig. 15 E. 



