EMBRYOLOaY.— THE OYUM. 673 



EMBRYOLOGY; 



on, DEVELOPMEXT OF THE FCETUS AND ITS ORGANS. 



It is proposed to bring tog-ether in the present Section a short state- 

 ment of the manner in which the parts of the body originate in the 

 embryo, and acquire by development in the course oi" foetal life their 

 complete form and structure. The collected facts bearing upon this 

 subject constitute the department of anatomy known as Embryology, 

 Embryological or Foetal Anatomy, or Foetal development, — a knowledge 

 of -which is not only most interesting in itself, but is also of grent 

 importance for the elucidation of adult human anatomy and the whole 

 science of Organic Morphology. 



Although much attention has been given to the structure of the 

 human foetus at different periods of its growth, yet the materials are 

 still wanting for a detailed history of its early development ; accord- 

 ingly it is necessary for further elucidation to have recourse to the 

 information obtained by studying the process in animals. But this 

 illustration from analogy is fully warranted by the general conformity 

 in the plan of development which has been ascertained to prevail 

 among the higher vertebrate animals, and by the agTeement with this 

 plan of the more important phenomena which it has been possible to 

 observe in the human species. In what follows, therefore, while the 

 main object will be to state the more important facts whicli have been 

 ascertained as to the development of the human ovum and embryo, the 

 history of the phenomena as they occur in birds and mammals will 

 also be referred to in so far as it tends to throw light on human 

 embryology. 



1. THE OVUM : ITS MATURATION, FECUNDATION AND SEGMENTA- 

 TION : FORI.IATION OF THE BLASTODERM. 



The mature ovarian ovum. — The unfecundated ovarian ovum 

 which is approaching maturity, and is about to be discharged from the 

 Graafian follicle and pass into the Fallopian tube, is composed of the 

 following parts, viz. : — 1. The firm, almost homogeneous external 

 vitelline membrane, which is termed zona pellncida in mammals ; 2. The 

 yolJc substance or vitellus, a mass of soft or semifluid protoplasmic 

 matter, involving numerous granules and oil globules, and containing 

 embedded in it, near the surface ; 3. The germinal vesicle, consisting 

 of a spheroidal delicate enclosing membrane with protoplasmic fluid 

 and fine granules within, and containing in its earlier states if not to 

 the last ; 4. The germinal sjwt or macula. 



The ovarian ovum, therefore, may be regarded as a complete organised 

 cell, in which the yolk forms the protoplasmic contents, the germinal 

 vesicle the nucleus, and the macula the nucleolus. 



Distinction of the germ.— In the ovum of the mammal tlie whole 

 yolk-substance is so uniformly of the same appearance and structure 

 throughout that, but for the presence of the germinal vesicle, little dis- 

 tinction can be perceived between one part and another ; and, further, 



