12 INTRODUCTION 



from unformed living substance. This theory, known as epigenesis, was proved 

 correct when, in 1827, von Baer discovered the mammalian ovum and later 

 demonstrated the germ-layers of the chick embryo. When, after the work of 

 Schwann and Schleiden (1839), the cell was recognized as the structural unit of 

 the organism, the ovum was regarded as a typical cell and, in 1843, Barry ob- 

 served the fertilization of the rabbit's ovum by the spermatozoon. Hence- 

 forth all multicellular organisms were believed to develop each from a single 

 fertilized ovum, which by continued cell-division eventually gives rise to the 

 adult body. In the case of vertebrates, the segmenting ovum differentiates first 

 three primary germ-layers. The cells of these layers are modified in turn to form 

 tissues, such as muscle and nerve, of which the Various organs are composed, and 

 the organs together constitute the organism, or adult body. 



Primitive Segments — Metamerism. — In studying vertebrate embryos we 

 shall identify and constantly refer to the primitive segments or metameres. 

 These segments are homologous to the serial divisions of an adult earth- 

 worm's body, divisions which are identical in structure, each containing a 

 ganglion of the nerve cord, a muscle segment, or myotome and pairs of blood- 

 vessels and nerves. In vertebrate embryos the primitive segments are known 

 as mesodermal segments, or somites. Each pair gives rise to a vertebra, to a pair of 

 myotomes, or muscle segments, and to paired vessels; each pair of mesodermal 

 segments is supplied by a pair of spinal nerves, consequently the adult verte- 

 brate body is segmented like that of the earth-worm. As a worm grows by 

 the formation of new segments at its tail-end, so the metameres of the vertebrate 

 embryo begin to form in the head and are added tailwards. There is this dif- 

 ference between the segments of the worm and the vertebrate embryo. The seg- 

 mentation of the worm is complete, while that of the vertebrate is incomplete 

 ventrally. 



GROWTH AND DIFFERENTIATION OF THE EMBRYO 

 A multicellular embryo develops by the division of the fertilized ovum to 

 form daughter cells. These are at first similar in structure and, if separated, any 

 one of them may develop into a complete embryo, as has been proved by the 

 experiments of Driesch on the ova of the sea-urchin. The further development of 

 the embryo depends (1) upon the multiplication of its cells by division; (2) upon 

 the growth in size of the individual cells; (3) upon changes in their form and 

 structure. 



The first changes in the form and arrangement of the cells give rise to three 



