2 INTRODUCTION 



formed animals existed in miniature in the egg, needing only the stimulus 

 of the spermatozoon to initiate development, or that similarly preformed 

 bodies, male and female, constituted the spermatozoa and that these 

 merely enlarged within the ovum. According to this doctrine of preforma- 

 tion 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) believed thatfhe 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 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. 



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

 the cell to be the structural unit of the organism, the ovum and spermato- 

 zoon were recognized as true cells. O. Hertwig, in 1875, was the first to 

 observe and appreciate the events of fertilization. Henceforth all multi- 

 cellular organisms were believed to develop each from a single fertilized 

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

 body, that of man, it is estimated, containing 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 com- 

 posed, 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 meta- 

 meres. 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 vessels and nerves. In vertebrate em- 

 bryos the primitive segments are known as mesodermal segments, or somites. 

 Each pair gives rise to a vertebra, to a pair of myotomes, or muscle seg- 

 ments, and to paired vessels; each pair of mesodermal segments is supplied 

 by a pair of spinal nerves, consequently the adult vertebrate body is seg- 

 mented 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 em- 

 bryo begin to form in the head and are added tailward. There is this 

 difference between the segments of the worm and the vertebrate embryo: 

 The segmentation of the worm is complete, while that of the vertebrate is 

 incomplete ventrally. 



