146 WEIGHT, SIZE, AND AGE OF THE HUMAN EMBRYO. 



those obtained from obstetrical clinics, where most of the material is from the last 

 three months of pregnancy. In such cases the records of weight can be safely relied 

 upon, but the measurements of the fetus are less accurate because they arc not 

 uniformly made, being done by many different and untrained observers and without 

 the aid of proper instruments. Comprehensive data for the later months of preg- 

 nancy, obtained through cooperation with the obstetrical department of the Johns 

 Hopkins Hospital, have been published by Meyer (191.",); his 2,394 cases were 

 white and negro in about equal proportions. The study of Zangemeister ( 191 1 ) is 

 also to be commended, particularly because of its convenient graphic presentation. 

 His paper includes the weight and length of the fetus and the weight of the various 

 organs, together witli the normal limits of variation, based upon averages obtained 

 from the literature and his own observations. For the earlier part of pregnancy 

 the studies of Mall (1910, 1918), though composite, contain the best data we have 

 on the size and age of embryos under 100 mm. long. 



The fact that in this laboratory there is a continuous accession of embryos of 

 all stages of development has made it possible to inaugurate a plan of systematic 

 measuring and weighing of each specimen by which the factors involved are kept 

 relatively constant. Toward this end the following precautions were observed 

 with respect to the selection, fixation, and measurement of the material: In the 

 first place, only normal specimens were used and these were classified in three 

 grades, depending on the condition in which they were received. Those that 

 showed no injury and in which the tissues were practically living at the time of 

 fixation were classed as grade 1. Those in which the preservation was not perfect 

 or which had been slightly injured in some way were classed as grade 2. Under 

 grade 3 were grouped the poorest specimens, including those thai showed some 

 maceration of the tissues or mechanical injuries, though where these conditions 

 were extensive enough to essentially alter the normal character of the specimen 

 it was not utilized for purposes of tabulation. These grades are illustrated in 



figure 1. 



FIXATION. 



Although in much of our material record was made of the weight and measure- 

 ments of the fresh specimen, many of the specimens were sent to us already fixed 

 in formalin and it was necessary, therefore, to accept the formalin state as a basis 

 for the group as a whole. Furthermore, since formalin is in general use in other 

 laboratories, this basis will facilitate the comparison of our observations with those 

 of other workers. It is to be remembered, however, when working with formalin 

 material, that the preservative introduces an artificial element which must be 

 taken into account. Young, fresh specimens, when placed in a 10 per cent for- 

 malin solution, quickly take up the fluid, becoming tensely distended, so that they 

 increase markedly in weight and to some extent in length. In older specimens, and 

 those in which the tissues are macerated, the distention is not so great. A speci- 

 men, after 1 icing distended by the solution, in the course of a few months tends to 

 gradually regain its original size and weight. Owing to the character of the sub- 

 cutaneous tissues of the scalp and head, the size of the head under these conditions 



