346 ZOOLOGY 



fertilization (as the albumen and shell in egg of birds). Fertil- 

 ization is external in most fishes and some amphibia, a,nd in- 

 ternal in the higher groups. Naturally it must be internal in 

 all forms in which large masses of food or shells come to sur- 

 round the ovum proper, since the sperm could not penetrate 

 these after they are once formed. Similarly internal fertilization 

 would be necessary in all cases where the young are developed 

 within the body of the mother. In all such there are special 

 organs and special instincts that lead to the introduction of 

 the sperm into the body of the female (copulation). This how- 

 ever must not be confused with fertilization. The uterus is a 

 special portion of the oviduct where early embryonic develop- 

 ment may occur. (See Figs. 205, 206.) 



362. Development. — Those eggs which are fertilized out- 

 side develop principally at the expense of the yolk of the ovum. 

 Those internally fertilized may receive, after impregnation, 

 additional materials for the further nourishment of the embrya, 

 as above noticed for reptiles and birds. The fertilized ova 

 may be retained for a longer or shorter time in the oviduct 

 or in some modified portion of it (uterus, in mammals) and 

 undergo development there. Where internal development is 

 slight (as in birds) or absent the animals are described as ovi- 

 parous; where it is considerable, as in mammals, and the young 

 are free at birth they are described as viviparous. 



The table on page 347 will give some ot the facts concerning the early develop- 

 ment of vertebrates. It will be found an excellent exercise for the students to 

 verify the data collected in this and the preceding table (p. 344). It can readily 

 be supplemented by a demonstration of figures from more advanced texts. 



363. The Muscular System. — We have seen above (§345) 

 that the internal layer of the mesodermic pockets comes to be 

 united with the digestive tract. This layer furnishes the non- 

 striped muscle fibres of its walls. The external portion, which 

 becomes associated with the ectoderm, gives rise to the muscles 

 of the body-wall and those which move the skeleton. The 

 fibres of these muscles are cross-striped or voluntary (Fig. 

 30). It is by means of them that locomotion is effected. The 

 skeletal muscles may be classed as (i) axial, and (2) appen- 

 dicular. The axial are well shown in Branchiostoma (Fig. 154) 

 and the fishes, where the whole body is made up of repeated 



