THE 48-HOUR EMBRYO 217 



sensory portion of the ear, which was simply a placode of cells at 33 hours, 

 has now become an ovoid cyst or otic vesicle. There is as yet no clear indica- 

 tion of the nasal placode, although ectodermal thickenings will be found in 

 cross sections. 



The tubular heart has bent in such a way that the atrium is anteriorly 

 located while the ventricle is posterior. The blood thus follows a spiral path 

 through the heart, beginning at the sinus venosus and passing through the 

 atrium, the ventricle, and the bulbus arteriosus and into the ventral aorta. 



Between the heart and the otocyst the branchial grooves may be seen. 

 There are three pairs present at this time. Three pairs of aortic arches connect 

 the ventral and dorsal aorta. Internally the thyroid gland and the liver appear 

 as small outgrowths of the gut. 



A comparison of the head with the posterior trunk region shows the 

 latter to be relatively undifferentiated. The eye and ear begin to differentiate 

 before all the somites have formed in the posterior region. Similarly a well- 

 developed foregut is present while the hindgut is just beginning to form. The 

 head fold of the amnion extends over the head and trunk region while the 

 tail fold is getting under way. The earlier development of anterior structures 

 is a common occurrence. The anterior parts of the pro-, meso-, and meta- 

 nephros develop first. 



The primitive streak is represented by a dense aggregation of cells just 

 in front of the tail fold. The remainder of the structure of the 48-hour 

 embryo can best be described in sections. 



As can be seen from Figure 134, the first transverse sections beginning at 

 the anterior end will pass through the mesencephalon. Next, the meten- 

 cephalon, mesencephalon, and diencephalon will be included in a single 

 section. Such a cross section is shown in Figure 135. The microscope gives a 

 reversed image, and right and left sides of the section are reversed. In addi- 

 tion to the divisions of the brain, note the mesenchyme around the brain and a 

 thin layer of epidermis enclosing the brain and mesenchyme. The amnion 

 completely surrounds the head section and is in close contact with the splanch- 

 nopleure at the bottom and the chorion at the top. The splanchnopleure 

 contains large blood vessels. 



The pituitary gland develops from the union of an epidermal pouch 

 called Rathke's pouch with the infundibulum of the diencephalon (Fig. 136) . 

 The cavity of the pouch is continuous with the stomodeum. Rathke's pouch 

 differentiates into the anterior lobe of the pituitary gland, whereas the 



