EMBRYO OF 12 MM. STUDIED IN SECTIONS. 



'He. Between the third and fourth aortic arches on either side is a small cavity 



med by entoderm, cl.III, a diverticulum from the third gill-cleft. Immediately 



>elow the otocyst is the anterior cardinal vein, Card. From a point below the 



ardinal there extends a prolongation, Hy, which may be taken as a portion of 



the hyoid or second branchial arch. It extends downward and consists of a mass 



of mescnchyma covered by ectoderm. It encloses a space, cl.II.ex, which may 



be regarded as the external portion of the second gill-cleft. In a neighboring 



;on (455) the prolongation of the pharynx shown in figure 187 can be traced 



farther until it opens into this space, cl.II.ex. The second cleft is open 



upon both sides of the embryo, the first and third have closing membranes, the 



fourth cleft is not yet so far developed that its entoderm has come in contact with 



the epidermis of the embryo. The second cleft probably always becomes open, 



differing in this respect from all the others. Why it has this peculiarity we do not 



know. The opening does not persist, but the exact history of its closure is at 



present unknown. The process, Hy, described as shutting in the external p >rtion 



the second gill-cleft has sometimes been termed the operculum, because it covers 



a gill-cleft opening, as does the operculum of a bony fish. In the lower part of 



our figure a portion of the somatopleure, Som, is shown where it extends ven- 



tralward to form the wall of the pericardial cavity. There is also included in the 



drawing a part of the pulmonary aorta, P.Ao. 



Pig Embryo of 12 mm. Studied in Sections. 



A pig embryo of 12 mm. has been selected as the center of study in this 

 because its anatomical relations are such that they may be readily grasped by 

 the student who has already studied 'the 'anatomy of an adult mammal, human 

 or other. At the same time the development of the organs is so advanced that 

 their fundamental relations may be observed. From an embryo of this sill 

 transition to the study of younger embryos is, even for the beginner, comp.i 

 easy. It is not necessary that the embryo should be of this exact size; ind( cl. 

 may be somewhat advantageous for the student to have an embryo a millirt 

 larger, or one, two, or even three millimeters smaller, since the figures and < 

 tions referring to the 12 mm. stage will enable him to identify all the striHtttB 

 to be found and yet call upon him for the exercise of care and judgment in ide 

 fying, from the data given in the following pages, the various parts in the q 

 what different stage he may be studying. Of 12 mm. pigs the author has ha<| 

 his disposal five good series belonging to the Harvard Embryological Collect^ 



The transverse series is the most important, and should form the bJ| 

 the study, and accordingly most of the sections figured are from su 

 Next in importance comes the sagittal series, but it is desirable that e 

 should have a series in the three standard planes at his disposal for *tn'|; 

 practical laboratory study each student should be required to make a 

 accurate camera lucida ^drawings of carefully selected 



