THE CLASSIFICATION OF THE EARLY STAGES. 113 



correlated development, so that generally an embryo at a given stage of develop- 

 ment in form will agree with its fellows in size; but to this rule there are not in- 

 frequently exceptions, and sometimes an embryo is found much larger than 

 others at the same stage. Moreover, the variability of embryos is very great, 

 for in specimens otherwise alike we find this or that organ advanced or retarded 

 in its development, as compared with the embryo as a whole. Nevertheless it 

 is possible with the information at command to determine with tolerable cer- 

 tainty the age of an embryo within two days plus or minus, up to the end of the 

 ninth week. For the course of development during the third week we possess 

 as yet no satisfactory data, but embryos of full three months are quite frequently 

 obtained, and are very characteristic in size and configuration (see page 89). 



The Classification of the Early Stages. 



Any attempt to divide embryos into stages must necessarily establish artifi- 

 cial groups, for in nature there is no demarcation. Division into stages is for 

 convenience, and ought, therefore, to be made by natural and obvious character- 

 istics. It seems to me that eleven stages may be conveniently discriminated, 

 as follows : 



First Stage. Segmentation of the Ovum: The general process is described 

 on pages 54 to 59. There are no observations upon this stage in man. 



Second Stage. Blastodermic Vesicle: The general development of the blasto- 

 dermic vesicle in mammals is described on page 60. Its development in man is 

 unknown. During this stage the embryonic shield is differentiated. An ovum 

 of a monkey in this stage is described on page 121; the single human ovum known 

 is described on page 123. 



Third Stage. Primitive Streak: No human ovum with a primitive streak 

 before the formation of the medullary plate has been observed. 



Fourth Stage. The Medullary Plate: In this stage there are several embryos 

 known. In all of them the amnion and chorion are already differentiated. 

 There is a large extra-embryonic ccelom. The chorionic vesicle is rounded and 

 somewhat flattened. In its greatest diameter it measures from 8 to 10 mm. It 

 is beset with short branching villi which were found present over the entire 

 surface, except in one case described by Reichert. The general relations are 

 indicated in the accompanying diagram (Fig. 54). The chorion has a distinct 

 epidermal and mesodermal layer and bears villi. To its inner surface is attached 

 the body-stalk which unites the embryo and chorion. From it springs the am- 

 nion covering tjie embryo, which measures only i.o to 1.5 mm., and from the 

 ventral surface of the embryo arises the yolk-sac, which is of rounded form and 

 about equal in diameter to the length of the embryo. 



