2 INTRODUCTION 



initiate development, or that similarly preformed bodies, male and female, con- 

 stituted the spermatozoa and that these merely enlarged within the ovum. 

 According to this doctrine of preformation all future generations were likewise 

 encased, one inside the sex cells of the other, and serious computations were made 

 as to the probable number of progeny (200 million) thus present in the ovary of 

 Mother Eve, at the exhaustion of which the human race would end! Dalenpatius 

 (1699) beheved that he had observed a minute human form in the spermatozoon. 



The preformation theory was strongly combated by Wolff (1759) who saw 

 that the early chick embryo was differentiated gradually from unformed hving sub- 

 stance. This theory, known as epigenesis, was proved correct when, in 1827, von 

 Baer discovered the mammahan ovum and later demonstrated the germ layers of 

 the chick embryo. 



About twenty years after Schleiden and Schwann (1839) had shown the cell 

 to be the structural unit of the organism, the ovum and spermatozoon were recog- 

 nized as true cells. 0. Hertwig, in 1875, was the first to observe and appreciate 

 the events of fertihzation. Henceforth all multicellular organisms were beheved 

 to develop each from. a single fertilized ovum, which by continued cell division 

 eventually gives rise to the adult body, containing, it is estimated, 26 million 

 million cells. 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, in the earth worm, are identical in structure, each containing a 

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

 sels 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 supphed by a pair of spinal nerves, consequently tlie adult verte- 

 brate body is segmented hke that of the earth worm. As a worm grows b}- 

 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 tailward. 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. 



