256 PRACTICAL DIRECTIONS. [APP. 



cut away the vascular area unless it is wanted for 

 examination. 



D. S^t,rface view of the transparent embryo. 



Since the embryo now lies on its side we shall 

 not have to speak of the view from above and below. 

 The views from the two sides differ chiefly as to the 

 appearance of the heart. 



The embryo {freed from the blastoderm and the 

 amnion) is to be floated on to a glass slide in the 

 usual way. It is necessary to protect it while under 

 examination, with a cover-slip, which must not be 

 allowed to compress it. To avoid this, we have found 

 it a good plan to support the cover-slip at one end 

 only, since by moving it about when thus supported, 

 a greater or less amount of pressure can be applied 

 at will to the object. 



The details which can at this stage be seen in a 

 transparent embryo are very numerous arid we re- 

 commend the student to try and verify everything 

 shewn in Fig. 24. Amongst the more important and 

 obvious points to be noticed are 



1. The increase of the cranial flexure and the lody- 

 flexure. 



2. The condition of the brain. The mid-brain now 

 forms the most anterior point of the head. 



The fore-brain consists of the inconspicuous 

 vesicle of the third ventricle and the two large 

 cerebral lobes. 



The hind-brain consists of a front portion, the 

 cerebellum with a thickened roof; and a hinder 

 portion, the fourth ventricle with a very thin and 

 delicate roof. 



3. Organs of sense. 



The eye especially is now in a very good state 

 to observe. The student may refer to Fig. 31, 

 and the description there given. . 



