NATURE 



489 



THURSDAY, SEPTEMBER 23, 1897. 



EXPERIMENTAL EMBRYOLOG\. 

 The Development of the Frog's Egg. By T. H. Morgan. 



Pp. X + 192. (New York : The Macmillan Company. 



London : Macmillan and Co., Ltd., 1897). 

 Ueber Verwachsungsversuche mit Amphibienlarz'en. 



Von Dr. G. Bom. 8vo. Pp. xi + 224. (Leipzig: 



Wilhelm Engelmann, 1897). 



THE theories of Roux and Weismann concerning the 

 significance of nuclear division have been the 

 cause of much useful work. The attempt to decide 

 experimentally whether the early divisions of a fertilised 

 ^%^ are in fact accompanied by a qualitative separation 

 of nuclear or other " determinant " material has led to 

 the remarkable observations of Roux, Driesch and others 

 upon the behaviour of isolated blastomeres, and upon 

 the effect of destroying one or more blastomeres in a 

 segmenting egg. Prof. Morgan has attempted to collect 

 the results of such observations in a form convenient for 

 students. His book contains a fairly full account of the 

 maturation, fertilisation and cleavage of the Frog's egg, 

 but the later stages of development are treated very 

 briefly indeed, the main purpose of the work being a 

 discussion of hypotheses such as those of Roux and 

 Weismann in the light of recent experiment. 



The difficulties in the way of a belief that the early 

 divisions of the eg% are accompanied by qualitative 

 separation of "determinant" substances are largely 

 increased by the demonstration that in any single 

 blastomere, removed from the &g^ at a sufficiently early 

 period, is capable of giving rise to an embryo which is 

 not appai^ently abnormal except in point of size. Prof. 

 Morgan devotes more than a third of his book to an 

 account of the experiments which lead to this conclusion, 

 and to the equally interesting experiments which show 

 that under certain circumstances the first two blastomeres 

 may remain in contact, and yet each of them may give 

 rise to a separate individual. The book is provided with 

 a useful bibliography. 



Whatever be the process by which the result treated 

 by Prof. Morgan is arrived at, there is no doubt that in 

 many animals each of the early blastomeres can by 

 proper treatment be made to produce an apparently com- 

 plete larva, while at a later stage in development the 

 removal of any group of cells involves the production of 

 a defective individual. Prof. Born has undertaken an 

 elaborate series of experiments in order to find out at 

 what stage of development this change in the properties 

 of embryonic tissues takes place. The first results of 

 his work, recently published in the Archiv fiir Ent7oick- 

 lungsmechanik, are now reprinted as a separate volume. 



Prof. Born has worked with the larvae of Amphibia, 

 and chiefly with Rarui esculenta and Bombittator tgneuSy 

 starting with unhatched larvae shortly after the closure of 

 the medullary groove. Such larvae may be removed 

 from their gelatinous cases for purposes of operation, 

 and reared in "normal" salt solution until the proper 

 hatching time, when they may be gradually transferred 

 to fresh water. After treatment of this kind, many of 

 NO. 1456, VOL. 56] 



Prof. Bora's larvae have metamorphosed into apparemly 

 healthy frogs. 



The first results are an extension and confirmation of 

 an old statement made by Vulpian, that the tail of a 

 newly-hatched tadpole will continue to differentiate after 

 being cut off from the body. The tail, cut away from an 

 unhatched larva, and grown in salt solution, continues to 

 exhibit histological differentiation and growth, with a 

 very slight amount of " regenerative " formation of new 

 tissue in front of the point of section. Dorsal and 

 ventral fin, medullary, tube, notochord, all the various 

 organs proper to the excised tail differentiate in a nearly 

 normal way, the principal abnormality due to the section 

 being the closure of the nerve tube and the growth of a 

 layer of ectoderm over the cut surface. The growth and 

 differentiation seem to go on normally until the yolk con- 

 tained in the tissues is used up, when the whole struc- 

 ture naturally dies of starvation. In the same way, the 

 excised head will continue to grow and to differentiate 

 until star\ation occurs, without regenerating the hinder 

 part of the body ; the amount of differentiation which 

 takes place before death being indicated by the state- 

 ment that in the oldest heads a complete chondrocranium 

 was formed. What is true of the head is true of seg- 

 ments cut out from the middle of the body : and as Prof. 

 Bom says : 



" Die Entwickelung jedes Organes, bis zur Schnitt- 

 flache, so gut wie bei der normalen Larve, fortschreitet, 

 mag die Schnittflache liegen, wie sie will. . . . Dabei ist 

 noch Folgendes zu bemerken. Rohrenformige Organe, 

 wie Ruckenmark, gehirn, Voraierengange, ja sogar 

 mitunter der Darm schlieszen sich an der Schnittflache 

 ab." 



An obvious defect of these experiments is that they 

 fail to show how far development would go, if a proper 

 arrangement for feeding the excised tissues could be 

 devised. Prof. Bom has, therefore, made a larger and 

 even more interesting series of experiments, by grafting 

 pieces, taken from the bodies of one set of larvae, into 

 the bodies of others. For example, the excised hinder 

 end of a larva is attached to the wound produced by 

 slicing away a small piece of the ventral surface of a 

 second larva. After a short time, the ectoderm of the 

 "accessory" tail unites with that of the body of the 

 "principal" tadpole, so that an unbroken ectoderm 

 covers the whole ; the nerve-tube of the " accessory '' 

 tail closes, and does not appear to develop a communi- 

 cation with the nervous system of the principal larva ; 

 the blood vessels of the stock anastomose freely with 

 those of the graft, so that the grafted tail has an 

 abundant food supply. In this way an organism is 

 produced, possessing two tails, and perhaps two pairs 

 of hind limbs. The rate of growth of the grafted tail, 

 &c., may not be the same as that of the corresponding 

 organs in the stock, so that one tail may after a short 

 time be larger than the other ; but the rate of histo- 

 logical differentiation is the same, so that if the limb of 

 the graft (as in one case described) remain much smaller 

 than that of the stock, it has not the structure of a limb 

 at an earlier stage of development than that of the stock, 

 but a reduced model of a limb at the same stage of 

 differentiation. This synchronism in the histological 

 differentiation is shown by Prof. Bom to occur whenever 



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