EMBRYOLOGY 



2884 



EMBRYOLOGY: THE SCIENCE OF LIFE 



J. Arthur Thomson, Regius Professor of Natural History, Aberdeen 



With the article on this department of the science of biology should 



be read those on Biology ; Evolution ; Life. See also Bacteriology ; 



Cell; Eugenics; Heredity, etc., and the biographies of Darwin and 



other biologists 



Embryology (Gr. embryon, em- 

 bryo ; logos, science) is the science 

 of the individual life-cycle, especi- 

 ally of the early stages during 

 which a germ grows into a body. 

 In other words, embryology is the 

 science of individual development. 

 From what looks like a minute 

 drop of living matter, though it is 

 doubtless a little world with intri- 

 cate organization, of which the 

 microscope gives us a hint, there is 

 gradually built up a young bird or 

 mammal, or some other creature, 

 whether animal or plant. Out of 

 apparent simplicity there arises ob- 

 vious complexity, and this develop- 

 ment is one of the most mysterious 

 processes in the world. 



In the great majority of cases the 

 individual life of plant or animal 

 begins in the intimate union of two 

 germ-cells or gametes, a more 

 passive egg-cell or ovum and a 

 more active sperm-cell or sperma- 

 tozoon. What follows refers chiefly 

 to animal development. 



EGG-CELL OR OVUM. In most 

 animals the egg-cells or ova are 

 formed by the multiplication of 

 primitive germ -cells within the 

 ovary. These primitive germ -cells 

 are the unspecialised descendants 

 of the fertilised egg-cell which de- 

 veloped into the body, and they re- 

 tain its essential qualities. They 

 increase in number by repeated 

 division, and some of them increase 

 in size and become unripe ova. It 

 frequently happens that an ovum 

 absorbs its less successful sister- 

 cells, or that they form a protec- 

 tive and often nutritive covering 

 (follicle cells) for it. In the ovary of 

 the fresh-water Hydra and the 

 marine Tubularia there is usually 

 only one ovum left out of many. 

 Formation of the Yolk 



The egg-cell usually accumulates 

 yolk-material, which may come to 

 be enormous in amount. The yolk 

 is furnished by the blood or other 

 nutritive fluid of the parent, the 

 follicle cells often acting as inter- 

 mediary units ; or it may be fur- 

 nished by special yolk-glands. It 

 accumulates in the ovum as glob- 

 ules or platelets mainly consisting 

 of a substance called lecithin. 



Around the egg are eventu- 

 ally formed sheaths or envelopes 

 of various kinds : (a) a delicate 

 vitelline membrane made by the 

 egg itself immediately after fertili- 

 sation ; (b) a follicular envelope 

 secreted by the follicle cells ; and, 



it may be, (c) a hard shell formed 

 by special glands in the walls of the 

 oviduct. The follicular envelope is 

 often formed before fertilisation, 

 and is perforated by a minute aper- 

 ture (the micropyle) or by several 

 of them. Otherwise the sperma- 

 tozSon could not effect entrance. 



The calcareous shells of birds' 

 eggs and the horny mermaid's purse 

 round the eggs of skates and dog- 

 fish are formed after fertilisation. 

 What is called a bird's egg consists 

 of a hugely dilated egg-cell heavily 

 laden with yolk, of a follicular en- 

 velope of some complexity around 

 this, of a large amount of albumen 

 or white of egg, of a tissue-paper- 

 like shell membrane, and of the 

 porous shell. Most ova are minute 

 spheres, and the majority are mi- 

 croscopically minute. The nucleus 

 of the unripe ovum is usually a 

 vesicle full of clear nuclear sap (the 

 germinal vesicle of the older embry- 

 ologists), with grains of readily 

 stainable chromatin material borne 

 on fibres crossing the sap, and a 

 dense corpuscle called the nucleolus. 

 Plant Egg-cells 



The egg-cell or oosphere of a 

 flowering plant lies within an " em- 

 bryo-sac," within the ovule, within 

 the ovary. When it is fertilised, 

 by a nucleus from the pollen-tube, 

 it divides and re-divides to form an 

 embryo. Thus the possible seed or 

 ovule becomes a real seed able to 

 germinate. 



THE SPERM-CELL OR SPERMA- 

 TOZOON. Primitive germ-cells, 

 usually localised in a special repro- 

 ductive organ or testis, increase in 

 number enormously and decrease 

 in size, becoming spennatocytes. 

 These divide further into unripe 

 spermatozoa or spermatids, which 

 become mature spermatozoa. A 

 typical spermatozoon is an ex- 

 tremely minute cell, hundreds of 

 which might be suspended in a 

 drop on a pin's head ; it is some- 

 times only about TO&roth of the 

 size of the egg-cell which is often 

 the size of a pin's head. In typical 

 cases it shows three parts : the 

 essential " head " consisting chiefly 

 of nucleus, the mobile " tail " 

 which is often fibrillated, and a 

 small middle piece between them 

 which bears a little corpuscle called 

 the centrosome. The spermatozoa 

 of threadworms and most crusta- 

 ceans are of a different type, slug- 

 gish and inclined to be amoeboid. 

 The spermatozoa of different ani- 



EMBRYOLOGY 



mals differ considerably in detail. 

 In flowering plants the male ele- 

 ment is represented by a generative 

 nucleus within the pollen-tube 

 which grows out of the pollen-grain. 

 In flowerless plants the male ele- 

 ment is usually a motile anthero- 

 zooid, as in ferns and mosses. 



MATURATION. In the history 

 of the germ- cells, both ova and 

 spermatoz5a, a remarkable process 

 occurs which is known as matura- 

 tion or reducing division. In the 

 nucleus of the immature stages of 

 the germ-cells there are a number 

 of readily stainable bodies or chro- 

 mosomes which separate from one 

 another, and can be counted when 

 division is going to occur. The 

 number is quite definite for each 

 species, e.g. 2 or 4 in the two races 

 of the threadworm of the horse, 

 12 in the grasshopper, 24 in the 

 mouse, and 24 hi man. This 

 definite " normal number," what- 

 ever it may be, is often (but not 

 always) demonstrable in all the 

 cells of the body as well as in the 

 germ-cells. 



In the mature ovum and the ma- 

 ture spermatoz5on the number of 

 these chromosomes is half the nor- 

 mal, therefore in one way or an- 

 other, at one stage or another for 

 it seems to occur variously in dif- 

 ferent types there is a process of 

 reduction, by which, in the division, 

 half of the whole chromosomes go 

 to each daughter-cell, whereas in 

 an ordinary cell-division each 

 chromosome is halved longitudi- 

 nally, and one set of halves passes 

 to each daughter-cell. The pecu- 

 liar kind of cell-division which 

 occurs during the maturation of the 

 germ-cells is called a reducing or 

 meiotic division ; the ordinary me- 

 thod is called an equation division. 

 Reduction of Chromosomes '' 



If we compare the chromosomes 

 with the wooden matches in a box, 

 a reducing division would be com- 

 parable to putting hah* of the 

 matches into another box, while 

 the equation division would be 

 comparable to splitting each match 

 longitudinally and then putting 

 one set of halves into another box. 

 When the egg-cell is 'ripening the 

 nucleus moves to the periphery and 

 gives off an abortive sister-cell, the 

 first polar body, which often 

 divides into two, but never comes 

 to anything. The nucleus of the 

 ovum then divides again and gives 

 rise to a second polar body, which 

 also comes to nothing. The re- 

 duction of the number of chromo- 

 somes is often effected in the for- 

 mation of the first polar body, and 

 there is a corresponding reduction 

 in the penultimate division of the 

 spermatocytes. 



In the sperm -cell lineage a 



