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TEXT-BOOK OF EMBRYOLOGY. 



reliable. This formula (for embryos from i to 100 mm.) was that the age 

 can be expressed by the square root of the length in millimeters multiplied 

 by 100 (Vlength in millimeters X 100). At present it can be said in general 

 that embryos of 2 to 3 mm. will fall within the fourth week of develop- 

 ment, embryos of 5 to 6 mm. will come about the end of the fifth week, those 

 of 10 mm. at the end of the sixth week, and those of about 25 mm. at the 

 end of the eighth week. The following table gives the approximate greatest 

 length at the ends of the lunar months following the second month: 



3d lunar month 70- 90 mm. 



4th lunar month 100-170 mm. 



5th lunar month 180-270 mm. 



6th lunar month 280-340 mm. 



7th lunar month. . . 

 8th lunar month . . . 

 9th lunar month. . . 

 loth lunar month. . 



350380 mm. 



425 mm. 



467 mm. 

 490500 mm. 



Some interesting observations have been made on the increase in volume 

 of the embryo (and foetus) and of its parts during development. It has 

 been estimated that the human ovum, which is about 0.2 mm. in diameter 

 weighs about four millionths of a gram (0.000004 gm.) and that at the end of 

 the first month the embryo weighs about four hundred ths of a gram (0.04 

 gm.). It thus appears that the human ovum increases approximately 

 10,000 times in weight during the first month of development. On the same 

 basis the relative growth rate which is the ratio of the gain during a given 

 period to the weight at the beginning of the period during the second 

 calendar month is 74, during the third month n, during the fourth month 

 1.75, during the fifth month 0.82, during the sixth month 0.67, during the 

 seventh month 0.50, during the eighth month 0.47 and during the ninth 

 month 0.45 (Jackson). The accompanying table taken from Jackson's 

 work gives the relative growth rate of the embryo in each of the ten lunar 

 months of pregnancy. 



