EMBRYOLOGY 385 



We have thus reached a stage of considerable importance 

 in which the three primary germ layers have been established : an 

 ectoderm on the outside, which also gave rise to the neural tube ; 

 an entoderm on the inside destined to produce the whole of the gut, 

 including the pharynx and a mesoderm, which from its commence- 

 ment included a hollow, the coelom, and is differentiated into 

 myotome, somatic and splanchnic portions. The further history 

 of the embryo of Amphioxus, while interesting, is fairly highly 

 specialised, and relates to the formation of the various structures 

 characteristic of the adult in particular, and not so much to 

 Chordates in general. The development so far may be considered 

 as that of a primitive Chordate, whose egg contains but little yolk, 

 and whose early stages are not complicated by the special require- 

 ments of the young embryo, and we may now pass on to see how a 

 similar stage is reached in other forms. 



Rana. 



The eggs of the frog when laid are spherical bodies about 

 2-3 mm. in diameter and surrounded by a thin coat of albuminous 

 matter composed of the tertiary egg membranes. As it floats in 

 the water it will be seen to consist of an upper or animal hemisphere, 

 dense black in colour owing to the presence of pigment in it, and a 

 lower or vegetative hemisphere, somewhat larger and white or greyish 

 white in colour. The egg is provided with a plentiful supply 

 of yolk and is telolecithal, that is to say, the yolk is concentrated 

 towards the vegetative pole. At the time the egg is laid its nucleus, 

 situated towards the top of the animal pole, is arrested in mitosis 

 preparatory to giving off the second polar body and so becoming 

 mature. The sperms are shed in the water where fertilisation occurs 

 and the entrance of the spermatozoon apparently supplies the 

 stimulus leading to the extrusion of the second polar body. After 

 the egg has been in water a short time the albuminous envelopes, of 

 which three can be recognised, absorb water and swell up until they 

 reach the size of a small pea. During the processes of formation, and 

 particularly -during the fertilisation and maturation of the egg, its 

 contents are so organised that the ovum as a whole is not merely 

 divided into poles, but has a definite bilateral symmetry which 

 determines- the direction of subsequent divisions. 



The first cleavage is holoblastic,. completely dividing the ovum 

 into two cells, and it is finished about two and a half hours after 

 fertilisation. The segmentation is indicated externally by the 

 appearance of a furrow starting in the animal pole and then running 

 completely around the egg, but the two cells remain close together, 



2 C 



