APP.] SURFACE VIEW OF EMBRYO. 243 



A thin ring of putty may now be placed round the 

 blastoderm, a small quantity of salt solution gently 

 poured within the ring, and the whole covered with a 

 glass slide, which may be pressed down until it is 

 sufficiently close to the embryo. The presence of any 

 air-bubbles must of course be avoided. 



Provided care be otherwise taken to keep the 

 embryo well covered with liquid, the putty riug and 

 the coverslip may be dispensed with. They are often 

 inconvenient, as when the embryo has to be turned 

 upside down. 



The object is now ready for examination with a 

 simple lens or with a compound microscope of low 

 objective. It is by far the best for the student to 

 begin at least with the simple lens. In order that 

 everything may be seen at its best, the slide should be 

 kept warmed to about 38°, by being placed on a 

 hot stage. 



D. Surface view of the transparent embryo from 



above. 



The chief points to be observed, are : — 



1. The head-fold. 



2. The indications of the amnion ; especially the false 

 amnion, or outer amniotic fold. 



3. The neural tube : the line of coalescence of the 

 medullary folds, the first cerebral vesicle, the com- 

 mencing optic vesicles, the indications of the second 

 and third cerebral vesicles, the as yet open me- 

 dullary folds at the tail end. 



4. The heart seen dimly through the neural tube ; note 

 its pulsation if present. 



5. The fold of the somatopleure anterior to the heart 

 (generally very faintly shewn). 



6. The fold of the splanchnopleure (more distinctly 

 seen) : the omphalo-mesaraic veins. 



7. The protovertebrce. 



16—2 



