CHORD ATA 345 



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 however must not be confused with fer- 

 tilization. The uterus is a special portion of the oviduct where 

 early embryonic development may occur. (See Figs. 205, 206.) 



362. Development. Those eggs which are fertilized out- 

 side develop principally by means of the yolk of the ovum. 

 Those internally fertilized may receive, after impregnation, 

 additional materials for the further nourishment of the embryo, 

 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 this internal develop- 

 ment is slight (as in birds) the animals are described as oviparous; 

 where it is considerable, as in mammals, and the young are 

 free at birth they are described as viviparous. 



The table on page 346 will give some of 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. 343). 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 and furnishes- the non-striped 

 muscle fibres of its walls. The external portion, which be- 

 comes 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 Amphioxus (Fig. 154) and 

 the fishes, where the whole body is made up of repeated seg- 

 ments (myotomes) of muscle fibres. The muscles segments 

 alternate with the segments of the spinal column, as one would 

 expect. The appendicular muscles are those which move the 



