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. Tin- e-- 

 aris.-s 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 disini 

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

 thin white yolk a mass occupying the centre of tin- CL^- is continuous 

 through a narrow isthmus with a tract lyin^ immediately hnirath 

 th.- j'-rminal disc (" Nucleus of Pander ") and this latter is prolonged 

 as a thin superficial layer over UK- surface of tin- e^Lf. I'd ween the 

 supi*rli<-ial layer and tin- central mass arc a number of thin con 

 f.-ntric layers of whit- yolk. 



T> .< ri^'lit 1 1 vary and oviduct \\liii-han- |>P .-rut in Mrly Stag6S UndeigO atrophy, 



niiiX' functional Tins i> |. rok-il.lv tu In- iv-;rdrd as .-in adapt i\ . arran^' 

 in-iit uliic-h lias IK-, -n <lc\ i-loji. d in Vri h Krai. > \\ith lar^f -^,U s t<>a\oid tin- dangers 

 wlii<-h \\oiild In- involvi-d in | li- synchronous passage Oil pair i>f r^-js ( >: 



,i!lv if Miitain.-d in a ii'jid -ln-11. into tin- narrow terminal |ioii:.ni ..f tin- 

 passage t<> i 



