Gastrula 135 



which is the aperture of invagination. If a hollow india- 

 rubber ball be pressed in with the finger until the two opposite 

 walls meet, some idea can be obtained of the Gastrula, as the 

 embryo in this stage is termed. Now, in the fowl we have a 

 groove formed, and with the formation of this groove coincides 

 the formation of, at least, the greater part of the mesoblast. 

 The mesoblast of Amphioxus is also established after the forma- 

 tion of the gastrula ; but in Amphioxus the invagination of a 

 layer of cells, at first ' external in position, is the way in which 

 the hypoblast is formed, this layer being, in fact, the invaginated 

 layer. In the fowl, as we have seen, the hypoblast is formed 

 before the groove is produced. This is a difficulty in the way 

 of considering the groove to be an abortive blastopore. The 

 typical gastrula, such as that of Amphioxus, resembles Hydra in 

 the number of points, which are — 



1 . It is two-layered. 



2 . The two layers are arranged in the form of a hollow sac. 



3. The sac communicates with the exterior by an aperture, 

 the mouth of the Hydra and the blastopore of the gastrula. 



It is thought that, in the embryo at the gastrula stage we 

 have a recapitulation of the HydrorX^o. form which was its 

 remote ancestor. 



Some other transitory organs will be more conveniently 

 dealt with in the following general account of the development 

 of the several systems. We may mention here, as being purely 

 transitory organs, the gill-clefts. It will be remembered that 

 the tadpole breathes by means of gills, which are vascular tufts 

 arranged as fringes along the margins of certain clefts, which 

 place the pharynx in communication with the outside world. 

 In the embryo chick there are four of these clefts formed, 

 which grow out from the hypoblast lining the pharynx, and 

 come into contact with the epiblast, a perforation being formed 

 at the points of contact. There are thus a series of apertures 

 established. These, it seems necessary to suppose, are the 

 exact equivalents of the gill-slits, though they never perform 

 the part of respiratory organs; nor, indeed, do they appear 

 to perform any function at all, which is a still further argument 

 in favour of their being comparable to the gill-slits of the frog, 



