x I ( )WL THIKD AND FOUKTH DAYS 547 



aninion. The Litter is now complete and closely invests the body 

 of the embryo. Lying loosely within the somatic stalk and of much 

 smaller diameter is the splanchnic or yolk stalk the continuation of 

 the splanchnopleure in a ventral direction as it passes out into the 

 wall of the yolk-sac. The body of the embryo has undergone a great 

 increase in size. The growth of its tissues has been particularly active 

 in its dorsal region and this has led to a continuation of the flexure 

 towards the ventral side which was already well marked in the third 

 dii y embryo. 



An important new feature in the fourth day embryo is provided 

 by the two pairs of limb rudiments each in the form of a dorsiventmlly 

 Hatteued ridge with rounded edge and broad base of attachment to 

 the body. The head of the embryo at once attracts attention by its 

 relatively enormous size. This is due to the relatively immense size 

 of the brain and eyes. We have here to do apparently with a case 

 of the precocious growth in size of organs which in the fully 

 developed condition possess extreme complexity of minute structure. 

 The main regions of the brain can be seen very distinctly: the 

 relatively large mesencephalon with its bulging dome-like roof, the 

 thalamencephalon with the pineal rudiment, the rapidly growing 

 rudiments of the hemispheres, and the hind-brain with its relatively 

 thin and membranous roof. The three main special sense organs are 

 all conspicuous the olfactory organ, the eye with its choroid fissure 

 and lens, the pyriform otocyst. Arranged in a row ventral to the 

 otocysts are the pharyngeal clefts three or four in number. In the 

 case of cleft I the ventral part of the cleft is becoming much narrowed 

 by the approach of its anterior and posterior walls. The dorsal end 

 of the cleft on the other hand remains dilated : it corresponds to the 

 spiracle of fish-like forms. 



The heart, which forms a large structure lying between the tip of 

 the head and the region of the fore limbs, is still in the form of a 

 coiled tube but the appearance of localized bulgings of its wall fore- 

 shadows its division into the various chambers characteristic of the 

 adult. Thus the curve of the tube lying posteriorly and on the right 

 is becoming dilated to form the ventricle : the part morphologically 

 in front of this leading towards the ventral aorta is slightly dilated to 

 form the conus arteriosus, while the curve lying anteriorly and on the 

 left side shows a slight bulging on each side foreshadowing the two 

 ,i u rides. Slight constrictions separate these various bulgings an atrio- 

 ventricular constriction narrowing the cavity to form' the auricular 

 canal, and a less conspicuous one between ventricle and conus. 



The general arrangement of the peripheral vessels is intermediate 

 between that of tin- third day (Fig. 241, A) and that of the fifth day 

 (Fig. 241, B) and need not be described in detail. Aortic arches I 

 and II undergo in turn a gradual process of obliteration while arches 

 IV and VI make their appearance farther back if they have not 

 already done so. It is also during this day that arch V makes its 

 brief appearance. 



