8 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



The model was also left in blocks of varying thickness, so that the structures not seen from 

 the outside might be followed, and it is possible thus to see the main points of the nervous, 

 alimentary, circulatory, and nephrogenic systems, and the mesodermic somites. 



For closer study, and also for use in illustrating this paper, special wax models were 

 made of the brain, heart, pharynx, and cloaca. Graphic reconstructions were made of the 

 nephrogenic system from the dorsal, medial, and lateral surfaces, using His's method of 

 plotting points on squared paper, and also a new and very instructive method described by 

 Buist (1913), known as contour reconstruction, in which an element of perspective is intro- 

 duced. Owing to the nature of this latter method there is, however, a considerable oblique 

 distortion which must be taken into account in interpreting the results. 



In addition to the above, the sections were gone over many times under the microscope 

 and the results of all methods have been correlated and incorporated as one in this paper. 

 The supplementary results appended in the description of each part, concerning Embryo V, 

 were all obtained by microscopic study of the sections and comparison of them with sections 

 of the same region in its twin, Embryo VI. 



AGE OF THE EMBRYOS. 



From the data given above it is difficult to determine accurately the age of these 

 embryos. Counting from the first coition after the last menstrual period, which gives the 

 earliest possible date of conception, December 20, to the last possible day of the embryo's 

 life, January 13, we get 24 days as the greatest age, and if conception were later and death 

 earlier the age will be several days less. The total length of Embryo VI, determined from 

 the serial sections, is 3.35 mm. and by applying Mall's (1903) rule that the age of the 

 embryo in days is equal to the square root of 100 times the length in millimeters, we get 18 

 days as the result. His (1880) estimates embryos of 3 to 4.5 mm. given length as being 

 2\ to 3 weeks old, and the ages for most embryos since described are based on these esti- 

 mates. It will be seen that the age of these embryos agrees with the estimates of His and 

 Mall fairly well. 



On the other hand, Eternod (1895) states the age of an early embryo described by him, 

 which measured only 1.3 mm., as 21 days, and he had very definite data upon which to 

 figure. The embryo described here is 3.35 mm. long, and of a degree of development far 

 advanced beyond Eternod's; yet, counting from the coition of December 20, it must be 

 accepted as 24 days or less in age, that is, at the most, only 3 days older than Eternod's. 

 The results are entirely contradictory as they stand, but it is quite possible that Embryo VI 

 is really much older than appears from the above facts, and if so, they will entirely agree. 

 It is a well-known fact that spermatozoa can live for weeks in the Fallopian tubes, which 

 practically act as receptacula seminis, and this embryo may be the product of fertilization 

 of an ovum from about the last menstruation by a spermatozoon from a coitus previous to 

 this period, and which would have been in the tube some days. If fertilization occurred 

 immediately after menstruation the embryo would be 5 weeks old; if during or just before 

 the flow, another half week would be added to the age. It would be quite possible for the 

 ovum to be fertilized just before menstruation and not be lost, as it would probably not 

 leave the tube and enter the uterus before the flow was over. Menstruation during early 

 pregnancy also is not unknown. If 5 weeks be the correct age it corresponds accurately 

 with figures given by Professor McMurrich (1913) in the last edition of "The Development 

 of the Human Body." Taking the ages of a few embryos where very exact information is 

 available, he says the ages of all early embryos have evidently been underestimated; an 



