134 



Elementary Zoology. 



Certain Transitory Organs.— In the course of development 

 of the fowl, as of other animals, various organs appear which 

 develop up to a certain point and then disappear. These can 

 be compared with organs found permanently in the lower 

 types, and their transitory existence is regarded as tending 

 towards the proof that the animals in which they are found in 

 the rudimentary condition are descended from animals like 

 those in which they are permanent structures. 



Some of these structures have been already referred to, as 

 was necessary in giving a general account of the early stages 



Fig. 71. — Formation of the gastrula of Amphioxus. (After Kowalevsky.) 



A, wall of the ovum, composed of a single layer of cells ; b, a stage in the _process_of 

 gastrulation ; c, completion of the process ; s, original or segmentation cavity 

 (blastocoel) of ovum : al, alimentary cavity of gastrula (archenteron); ect^ outer layer 

 of cells ; ent, inner layer of cells ; 0, orifice, constituting the mouth (blastopore). 



in the development. Thus the primitive streak is by most 

 persons looked upon as the equivalent of the blastopore of 

 other embryos, and as the equivalent of the mouth of coelenterate 

 creatures, such as Hydra. In many animals — for instance, in 

 Amphioxus — the embryo, after a number of cell divisions,' be- 

 comes a one-cell-walled hollow sac, the cavity being the blasto- 

 coel, equivalent to the similarly named cavity of the chick 

 embryo. At one point this wall is thrust in, as is shown in the 

 accompanying figures, thus producing a double-layered sac with 

 a new central cavity and an aperture leading to the exterior, 



