CHAP. XLI.] OF THE FCETUS. 595 



of the necessary transitional changes. The process of growth is a 

 simple one, and consists merely of the appropriation of new material 

 by structures already formed. The further development of the 

 different textures gradually takes place, and the various organs 

 assume their permanent character, until about the seventh month 

 the foetus has attained a length of sixteen inches or more, and 

 under favourable circumstances, its life may be preserved if it be 

 born at this early period. The testicles descend about the eighth 

 month. 



By the end of the ninth month the fretus has attained the length 

 of eighteen or twenty inches. The head is covered with hair, and 

 the skin becomes invested with a soft pultaceous substance, the 

 vernix caseosa, which consists of cells of epidermis, with a consi- 

 derable quantity of oily material. The membrana pupillaris is 

 absorbed. During the latter months of pregnancy, the child lies 

 in utero, with its head downwards, the position in which birth takes 

 place. 



The student is referred, for further information upon the subjects treated 

 of in the present chapter, to the works enumerated at the end of chapter xl., 

 and to the following : Reichert's Observations on the Development of the 

 Chick, in Muller's Physiology ; translated by Dr. Baly. Bischoff's Mono- 

 graphs on the Development of the Dog and Guinea-pig. De Graaf, Opera 

 Omnia. Von Baer's Entwickelungs-geschichte. Dr. Thomson, in the Edin- 

 burgh Med. and Surg. Journal, No. 140. Wagner, Icones Physiol. Article 

 Ovum, in the Cyclopaedia of Anatomy and Physiology, by Dr. Allen Thomson. 



