THE HUMAN EMBEYO AT DIFFEKENT PEEIODS. 79 



foetus, from vertex to heels, is 25-27 cm. (10-10- in.), from vertex to coccyx 20 cm. 

 (8 in.), and its average weight is about half a kilogramme (1 T V Ibs.). 



Sixth Month. The skin is wrinkled and of a dirty reddish colour. The hairs 

 are stronger and darker. The deposit of sebaceous matter is greater, especially in the 

 axillae and groins. The eyelashes and eyebrows appear. At the end of the month the 

 total length of the foetus, from vertex to heels, is from 30-32 cm. (12-12f in.), and its 

 average weight is about one kilogramme (2i Ibs.). 



Seventh Month. The skin is still a dirty red colour, but it is lighter than 

 in the previous month. The body is more plump on account of a greater deposit of sub- 

 cutaneous fat. The eyelids re-open, and the foetus is capable of living if born at this 

 period. Its total length at the end of the month, measured from vertex to heels, is 35-36 

 cm. (14-14f in.), and its weight is about one and a half kilogrammes (3J Ibs.). 



Eighth Month. The skin is completely covered with sebaceous deposit, which 

 is thickest on the head and in the axillae and groins,, and its colour changes to a bright 

 flesh tint. The umbilicus is farther from the pubis, but it is not yet at the centre of the 

 body. The total length of the foetus, from vertex to heels, is 40 cm. (16 in.), and its weight 

 varies from 2 to 2J kilogrammes (4J-5| Ibs.). 



Ninth Month. The hair begins to disappear from the body, but it remains 

 long and abundant on the head. The skin becomes paler, the plumpness increases, and 

 the umbilicus reaches the centre of the body. At the end of the ninth month, when the 

 foetus is born, it measures about 50 cm. from vertex to heels (20 in.), and it weighs from 

 3-3 kilogrammes (6^-7^- Ibs.). 



The age of a foetus may be estimated, approximately, by Hasse's rule, viz., Up to the fifth 

 month the length in centimeters, the lower limbs being included, equals the square of the age 

 in months, and after the fifth month the length in centimeters equals the age multiplied by five. 



NOTE 1. Evidence is gradually accumulating which tends to show that the reduction of the number of 

 chromosomes may take place during the last divisions of the germ mother cell, that is before the growth of 

 the oocyte or spermatocyte I commences, and therefore before maturation commences. 



NOTE 2. There is evidence which points to conclusions somewhaf different from those stated on p. 14, 

 regarding the dentoplasm in mammalian ova, but it is not yet sufficient or sufficiently conclusive to justify its 

 incorporation in a text-book account. 



NOTE 3. The recent observations of G. Fineman, Anat. Hefte, 159 H. (53 B. H.), 1915, show that the 

 ductus endolymphaticus is not derived from the original canal of communication with the exterior, but is 

 formed independently by a process of evagination. 



NOTE 4. Evidence which has accumulated since this statement was made tends to show that blood 

 corpuscles and the endothelial cells which form the walls of the primitive blood-vessels are derived from 

 different ancestors, the endothelial cells from mesenchyme cells, and the red blood corpuscles form angioblasts 

 which may be derived, as some observers believe, from mesenchyme cells, or, as others think more probable, 

 from entoderm cells. 



NOTE 5. The origin of the white blood corpuscles is still uncertain ; according to some investigators they 

 and the red corpuscles have common ancestors and the same ancestors may produce endothelium also ; this 

 is the so-called monophyletiq view. It appears probable, however, that, in some vertebrates, the white 

 corpuscles are derived from one set of mesoderm cells, the red corpuscles from another, and the endothelium 

 of the blood-vessels from a third set of mesodermal cells, each set of mesoderm cells being capable of pro- 

 ducing only one kind of descendant ; tliis is the polyphyletic view. 



