158 EMBRYOLOGY. 



into a somewhat stouter fibre, which is streaked longitudinally, 

 like the ordinary epithelial element, and like it, bears at its 

 extremity a bunch of cilia ; but, as has been already said, exam- 

 ples are met with, in which the special peculiarities are wanting. 

 In the frog, the processes of the epithelial elements appear 

 to penetrate the mucosa, so as to form a network of fine trabe- 

 culrc. The finest branches of the olfactory nerve are seen to 

 tend towards this network, but have not been traced into actual 

 continuity with the extremities of the so-called olfactory cells. 

 The mucosa and its glands must be studied in sections. 



CHAPTER XIII. 

 EMBRYOLOGY. 



IN treating of the methods which are commonly employed 

 in the study of general embryology, we shall follow the same 

 plan as in special histology ; noticing only those points which 

 are of importance to the beginner. 



As is well known, three parts are distinguished in every 

 mature egg: the vitelline membrane, the yolk or vitellus, and 

 the germ. The last-mentioned is the essential part, and as- 

 similates itself to the general idea of the cell, viz., an organism 

 composed of protoplasm, which possesses the capability, under 

 certain conditions, of performing amoeboid movements. In 

 the protoplasm of the germ the germinal vesicle, a body 

 analogous to the nucleus of other cells, is embedded; and 

 within this lies the germinal spot, the analogue of the nucleo- 

 lus. According as the two elements of the egg, which are 

 inclosed by the vitelline membrane, viz., germ and yolk, exist 

 separately from one another, or form a single body, eggs are 

 subdivided into two large groups, viz., meroblastic eggs, in 

 which the germ is separate from the yolk such as those of 

 the bony fishes, scaly reptiles, and birds ; and holoblastic eggs, 

 in which the germ itself contains the elements of the yolk 

 those of the cartilaginous fishes, amphibia, and mammals. 



In eggs of the first group, the germ lies upon the yolk in 

 the form of a disk; for which reason it receives the name of 

 blastoderm : formerly it used also to be termed (after Reichert) 

 "formative yolk," while the yolk itself was called "nutritive 

 yolk." The first process that claims the attention of the 

 embryologist is cleavage. The fertilization of the egg sets 

 this process going. It is called cleavage because the germ 

 divides into two cleavage masses, each of these again into two, 



