306 



EMBRYOLOGY. 



breadth posteriorly, and terminating in two (fig. 337, d,f) 



or three (fig. 337, e) crura ; these 

 are the future great venous 

 trunks, which as yet are lost in- 

 sensibly in the germinal mem- 

 brane. Even at this period un- 

 dulating motions, rhythmical 

 contractions of the heart, may 

 be perceived, by which the 

 somewhat wavy appearance of 

 the organ is produced ; the same 

 clear or nearly colourless fluid 

 is in motion in the heart as in 

 the vessels in the periphery. 

 The heart occupies the whole 

 space from the involucral point 

 of the germinal membrane to 

 the cranial end of the embryo, 

 and is consequently, when the 

 embryo is contemplated from 

 below, covered by the part of 

 the serous membrane which at 

 the same time forms the involu- 

 crum capitis. The embryo, 

 which at the end of the first day 

 bore some resemblance to a punt 

 or flat-bottomed boat, by the 

 middle of the second day has acquired the form of an ordi- 

 nary small boat turned over, the sides of which (the ventral 

 laminae) converge, whilst the head is much curved or beak- 

 fashioned (the bending down of the head), and furnished 

 with a particular cover (the involucrum capitis) ; the pos- 

 terior part is also somewhat recurved, but much less so than 

 the anterior part, by the commencing development of the 

 caudal envelope. The ventral channel extends from the pos- 

 terior margin of the heart (fig. 337) to the crescentic plait of 

 the caudal envelope (fig. 334, from e to g, seen through the 

 back of the embryo). 



[ 486. The changes that occur during the second half of 

 the second day, from the thirty-sixth to the fiftieth hour, ure 

 the following : the dorsal laminae are closed along the whole 



Fig. 337. Anterior end of an 

 embryo scarcely of greater age 

 than that of fig. 330, seen from 

 the abdominal (the vitellary) as- 

 pect, to show the first formation 

 of the sacculate heart, a, with 

 its immerging vascular (venous) 

 trunks, d, e, f ; b, b, crests of 

 the laminae dorsales seen shining 

 through. 



