204 GENEEATION AND DEVELOPMENT. 



believed that the germ spot or disc (cicatricula) was only a 

 trace of the attachment of the yolk to the ovary, and that 

 the chalazse constituted the material of the embryo, being 

 the main cause of embryonic development (prcecipua causa 

 pulli generationis) after having been impregnated,* and that 

 Harvey seems to have believed that embryonic development 

 began in the broad end of the egg.i 



Even to-day, many people believe that the chalazse are 

 what they call the &quot; life &quot; of the egg, and this represents to 

 their minds the parts where development begins. 



Aristotle says that the first signs of development are 

 noticeable after three days and nights, the heart being 

 visible as a palpitating blood-spot whence, as it develops, 

 two blood-vessels, which wind about, extend to the 

 surrounding tunics, and a membrane with threads of blood 

 encloses the whole, away from the aforesaid blood-vessels. 

 A little later, he continues, the body of the embryo, quite 

 small and white, is seen, the head being distinct and the 

 eyes very prominent or conspicuous, while the lower parts 

 of the embryo are not in proportion to the upper parts. 

 One vessel from the heart leads to the enveloping membrane 

 and the other to the yolk, after the manner of an umbilical 

 cord. The development of the young bird, he says, com 

 mences from the white, and its nutriment is derived from 

 the yolk, through what is equivalent to an umbilical cord. J 



Such is Aristotle s description of the development of a 

 chick, from about the fourth day to about the eighth day, 

 judging from the appearances he describes. It is now known 

 that development of the embryo commences in the germinal 

 spot or disc, situated on one side of the yolk. In consequence 

 of the yolk opposite the germinal spot being denser than that 

 on the side of the spot, this remains uppermost, however 

 the egg may be rotated by the sitting hen, the yolk being 

 steadied by the chalazse. During the early stages of develop 

 ment, the embryo is in process of being constricted off from 

 the yolk, and a bulging is noticeable, although, on account 

 of an apparent sinking in of the embryo, the bulging is only 

 slight. The time at which signs of development are first 

 seen by the unaided eye depends not only on the acuteness 

 of vision of the observer, but also on the extent of his know 

 ledge of embryology, but it may be said that signs may be 



* De Formatione Ovi et Pulli, Padua, 1625, pp. 24, 34, and 48. 

 f Exercitat. de Gener. Anim., 1680, p. 64. 



