514 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



now widely separated from the embryonic body by secreted amniotic 

 fluid. If the embryo is a well-advanced one towards the end of the 

 sixth day the amnion, which is now muscular, may exhibit periods of 

 muscular contraction during which the embryo is rocked to and fro 

 in the amniotic fluid. These movements must be distinguished from 

 the occasional contractions of the muscles of the embryonic body 

 which also occur about this time though they are much less 

 conspicuous. 



After a careful study of the living embryo with the allantois and 

 yolk-sac hanging from its ventral side it may be excised along with 

 a circle of vascular area, floated into a watch-glass and examined 

 alive with a lens or binocular, and then treated with fixing fluid such 

 as Bouin's solution. The latter brings out the surface modelling which 

 should be carefully studied especially in the region of the gill clefts. 



Dissect off the amnion, add more fixing fluid and after superficial 

 fixation renew the Bouin's solution. It is a good plan to suspend the 

 embryo by the yolk-sac so that the weight of the head causes the 

 neck to become somewhat straightened. After the embryo is 

 sufficiently fixed the neck may be cut through and the lower surface 

 of the head studied for the relations of the olfactory rudiments and 

 mouth. 



Sagittal sections through the head are particularly instructive 

 at this stage. 



VIII. Segmentation. — To obtain segmentation 'stages hens which 

 are regular layers should be chosen. In such cases the egg is laid at 

 a slightly later time on consecutive days. As a rule egg-laying is 

 confined to the forenoon and early afternoon and when an egg is due 

 after the end of this period it is retained within the oviduct and not 

 laid until next day. The retention of an egg in this way inhibits 

 the process of the ovulation so that a new egg is not shed from the 

 ovary until the preceding one has been laid. 



History of the ' Egg up to the Time of Laying. — The egg 

 arises as a single cell of the left ovary 1 which grows to a relatively 

 enormous size as yolk is deposited in its cytoplasm. The yolk is of 

 a characteristic yellow colour but in particular tracts the disintegra- 

 tion of its granules into finer particles gives it a white colour. Of 

 this white yolk a mass occupying the centre of the egg is continuous 

 through a narrow isthmus with a tract lying immediately beneath 

 the germinal disc (" Nucleus of Pander ") and this latter is prolonged 

 as a thin superficial layer over the surface of the egg. Between the 

 superficial layer and the central mass are a number of thin con- 

 centric layers of white yolk. 



1 The right ovary and oviduct which are present in early stages undergo atrophy, 

 never becoming functional. This is probably to be regarded as an adaptive arrange- 

 ment which has been developed in Vertebrates with large eggs to avoid the dangers 

 which would be involved in the synchronous passage of a pair of eggs of great size, 

 more especially if contained in » rigid shell, into the narrow terminal portion of the 

 passage to the exterior. 



