270 



SCIENTIFIC NEWS. 



[Mar. 23, 18 



arrangement, being wound round and round the yolk-sac. 

 It is said to be possible to unwind the white completely 

 in a hard-boiled egg, a feat which we have failed to per- 

 form, but it is easy to prove that the spiral winding 

 actually exists. The yolk is enclosed in a transparent 

 spherical bag, the yolk-sac. It consists of several con- 

 centric layers, which can be demonstrated, though not 

 easily, in a hard-boiled egg. At one point on the surface 

 a whitish patch is seen. Here is situated the so-called 

 " white yolk," a substance of slightly different composi- 

 tion and lower specific gravity than the rest of the yolk. 

 It acts as a float, and keeps uppermost the spot where 

 the body of the chick is to form. 



The history of the egg begins long before it is laid. 

 It leaves the ovary as a soft, round bag, containing only 

 yolk, and then becomes invested, during its passage along 

 the oviduct, with the successive layers of white, shell- 

 membrane and shell. Fertilisation occurs in the upper 

 part of the oviduct, and while the shell is forming, the 

 yolk undergoes the singular internal change known as 

 segmentation, the first formation of cellular tissues by 

 division of the originally simple cell. While hatching 

 goes on, the yolk-sac rises to the top in all positions of 

 the egg, and therefore lies close to the body of the hen, 

 being covered only by the shell and a scanty layer of 

 albumen. The spot of white yolk immediately above 

 which the body of the chick forms, occupies the highest 

 point of the yolk-sac, no matter how the egg may be 

 turned. At first the tiny chick lies face downwards upon 

 the yolk, one side being turned towards the broad, the 

 other towards the narrow end of the shell. As it gets 

 a little bigger, its head topples over to one side. Still 

 later it shifts, so that its body lies along the principal 

 diameter of the egg. When hatching is ended, the chick 

 is curled up in a very close and apparently uncomfortable 

 position, and almost fills the whole space which was 

 originally occupied by the yolk and white. 



The sitting hen leaves the eggs to cool once or twice a 

 daywhileshe goes away to feed. Aboutonce ada> she turns 

 the eggs with her foot. These points have to be noticed 

 in artificial hatching. If the eggs are not turned, a certain 

 temporary breathing organ, the allantois, is apt to adhere 

 to the shell, membrane, or to some part of the body, 

 causing injury and even death. Artificial incubators 

 should provide for free access of air, a fair amount of 

 moisture, and a tolerably steady temperature of 41° C. 

 (106° F.), which must not be exceeded. Hatching 

 occupies three weeks in the case of the fowl. Fertile 

 eggs are only to be had in sufficient numbers in spring. 

 From the end of February to June they are plentiful 

 enough. If the reader has a " broody " hen, or can fit 

 up a water-bath with a regulated gas-jet or other contriv- 

 ance for securing the necessary conditions to successful 

 hatching, he may observe very easily and pleasantly 

 the principal stages of the development of a chick, a study 

 which has no match in point of biological interest. The 

 eggs should be marked with the date of setting, and taken 

 out at different times. The egg of the third day is easy 

 to manipulate and very instructive, and it is not a bad 

 plan to get quite familiar with it before attempting either 

 earlier or later stages. Have ready some water of the 

 temperature of the hand (warmed normal saline solution 

 is better still), prick a hole in the broad end of the egg, 

 so as to allow the escape of air from the air space. This 

 will cause the yolk-sac to sink a little, and escape injury 

 in opening. Then cut round the egg-shell with a stout 

 pair of scissors, floating all the contents at once into the 



warm water. A simple lens will do a good deal for the 

 investigation of the embryo, and those who have access 

 to Malpighi's wonderful treatise on the formation of the 

 chick, published more than 200 years ago, can see in his 

 elaborate and accurate figures how much may be done 

 without compound microscope or staining or microtome. 

 For thorough study, however, the microtome is an 

 immense help, and some technical book, such as Foster 

 and Balfour's " Manual of Embryology," should be dili- 

 gently studied. 



It may give the beginner some notion what to look for 

 if we describe briefly a three days' chick. At this 

 stage the body lies in the middle of a circular area over- 

 spread by blood vessels. Two arteries leave the hinder 

 end of the body, and ramify upon the yolk-sac. Here 

 the blood takes up nutriment and a fresh supply of 

 oxygen, returning by two veins which pass to the heart 

 a little in front of the arteries just mentioned. The heart 

 is a rounded speck, as yet outside the unclosed body- 

 wall. It beats energetically, driving the blood in the 

 first instance to the neck along the arterial trunk. This 

 arterial trunk divides on each side into three aortic 

 arches, which encircle the gullet, and re-unite above it 

 to form the dorsal aortse, from which, among others, the 

 vessels of the yolk-sac are given oif. The two dorsal 

 aortae are now united for a short distance. In this stage 

 the vessels of the chick resemble in a striking degree 

 those of a full-grown fish. The arterial trunk in each 

 animal breaks up into several successive pairs of aortic 

 arches, which encircle the gullet and re-unite above. In 

 the fish, gill-slits intervene between the arches, and the 

 arrangement is obviously adapted for securing the aera- 

 tion of the blood. A stream of water entering by the 

 mouth traverses the gullet and issues by the gill-slits, 

 bathing the blood-vessels which lie spread out in 

 its course. In the embryo chick we have substan- 

 tially the same arrangement of vessels and gill-slits, 

 but the stream of water upon which the imme- 

 diate use of the whole apparatus depends has ceased 

 to flow. This is plainly one of those survivals of 

 primitive structure which so often force themselves upon 

 the notice of the emhryologist. The brain occupies the 

 curved fore end of the chick, and its lobes are tolerably 

 plain. The eye and ear are easily made out. At present 

 the head is lying on its side, but the hinder part of the 

 body retains its original position, what is afterwards to 

 form the front of the abdomen being still open towards 

 the yolk-sac. On each side of the recently closed tube 

 which represents the spinal cord, is a row of " meso- 

 blastic somites," out of which the paired muscles of the 

 back and sides, together with some other regularly 

 repeated parts, will shortly be developed. No sign of the 

 limbs is to be discerned, but in the course of the fourth 

 day these will begin to protrude. A great part of the 

 body, especially at the head end, is enveloped by a 

 transparent hollow sac filled with fluid. This is the 

 amnion, a temporary structure which does not enter 

 into the permanent body. 



The figure will explain these and other details, and 

 may be useful in guiding the first steps of some aspirant 

 to the fascinating pursuit of embryology. 



Subterranean Moss. — According toProfessorSchnetzbr, 

 a curious moss is found, growing at the depth of 

 200 feet, in the sub-lacustrine moraine of Yvoire. It 

 contains grains of chlorophyll perfectly formed. 



