EXTERNAL FORM OF EMBRYO OF 7.5 MM. 221 



Selection of the Stages. The most profitable stage to study is that of an embryo 

 of from ii to 13 mm. in length. Each student should have three specimens 

 of this stage, and it is advantageous that the specimens given each student be 

 approximately of the same size. The embryos ought to be first studied carefully 

 as to their external form and then cut into serial sections in the transverse, sagit- 

 tal, and frontal planes. Of these, the transverse series forms the principal basis 

 of study, and the other series are to be used principally to clear up the student's 

 conception of the relation of parts. Embryo pigs of the size specified have the 

 typical class characteristics of mammalian embryos, and may readily be distin- 

 guished from the embryos of any other class of vertebrates. The differentiation 

 of the anlages of all the important organs is accomplished, so that these anlages 

 can be identified with certainty and their genetic relations to the adult structures 

 can be clearly grasped by the student. At the same time, although the ana- 

 tomical differentiation is well advanced, the histological differentiation has made 

 very little progress, hence the embryos in question are particularly instructive to 

 beginners. The anatomy of the pig at this stage is, therefore, readily understood 

 by the student who knows the general anatomy of the adult. Older embryos 

 are more complicated and yield such long series of sections that the beginner 

 is apt to be discouraged. Younger embryos, owing to their spiral twisting, are 

 exceedingly difficult for students" to understand when sectioned. After having 

 thoroughly mastered the structure of the pig embryo of from 11 to 13 mm., the 

 student may advantageously extend his study of embryos to other sizes. If, as 

 is done in this work, the principal study is made with embryos of 12 mm., the 

 student may proceed to make sections of other stages as follows: 



Pig embryo of 6 mm., transverse. 



Pig embryo of 9 mm., transverse and sagittal series. 



Pig embryo of 17 mm., transverse series. 



Pig embryo of 20 mm., transverse and sagittal series. 



(Of the head alone, the frontal series.) 

 Pig embryo of 24 mm., of the head alone, frontal series. 



The Study of the External Form. 



The student should make a careful and thorough study of the external form 

 of every embryo, and make, with the aid of the camera lucida, an exact drawing 

 of every embryo before he cuts it into sections. He will soon learn that such a 

 drawing is almost indispensable for the interpretation of the sections. 



In the following paragraphs, embryos of 7.5, 10, 15, and 20 mm. are figured 

 and described from specimens which have been hardened in Zenker's fluid and 

 preserved in alcohol. The description of these stages will be sufficient to enable 

 the student to understand any of the embryos he is required to study. 



Pig Embryo of 7.5 mm. (Fig. 165). The student maybe helped in the iden- 

 tification of parts by comparison with figure 166, which has explanatory lettering. 



