T8 



SCIENCE 



[Vol. LVI, No. 1438 



one member essentially without a sac, as such. 

 This conclusion was based on the following 

 facts : 



1. There is no external evidence of a second 

 yolk sac or stalk although the most careful 

 search was made for them. 



2. The umbilical cord lacks a yolk-stalk com- 

 ponent, as proved microscopically by serial 

 sections. 



3. The single yolk sac shows no indication 

 of a second stump, nor are its vessels sugges- 

 tively arranged as if at any previous time in 

 relation to a second stalk. 



4. Although the yolk stalk normally be- 

 comes separated from the gut in embryos 

 slightly younger, its connection with the yolk 

 sac is retained until later. (On this point 

 Professor Lewis's criticism unintentionally car- 

 ries the erroneous implication that it is even 

 remarkable that the other stalk had retained 

 its connection with the sac until this period — 

 and hence the early disappearance of one is 

 entirely obvious!). 



5. The yolk stalk, with its vessels and invest- 

 ing tissue, usually is recognizable until a con- 

 siderably later period than the six weeks' em- 

 bryo in question; Minot records that it persists 

 beyond the fourth month but seems to have 

 disappeared by the sixth; Lonnberg states that 

 portions of its vessels may be found rarely at 

 birth; and in any case they are easily demon- 

 strable in embryos five or more times the size 

 of my specimen. 



Evaluating these several points I was led to 

 favor an early primary separation, rather than 

 a late secondary one with the coincidence of 

 precocious disjunction of a stalk and its simul- 

 taneously precocious disappearenee. After 

 thorough reconsideration I still incline to the 

 same opinion though recognizing fully the 

 possibility of the alternative interpretation 

 which I myself had considered but too sum- 

 marily dismissed without mention. But which- 

 ever interpretation is correct, the real objective 

 of the communication is equally supported, for 

 both refer to a single-ovum origin. 



In the further discussion of this specimen 

 several deductions were drawn as to the physi- 

 ological import of absence of the yolk sac. No 

 implication of morphological development, ex- 



cept mention of the ingi-owth of bloodvessels, 

 was meant, and I supposed the context made 

 this clear; if not, several statements must have 

 seemed as revolutionary to others as they did 

 to Professor Lewis. When, therefore, I spoke 

 of the yolk sac as "not essential to the growth 

 of an embryo or the proper differentiation of 

 its parts," I was merely referring to the 

 "growth" (that is, increase in size) of an em- 

 bryo and its organs, and the coincident "dif- 

 ferentiation" (or orderly progress) of its 

 developing parts. The sole aim was to draw 

 attention to the physiological insignificance of 

 the yolk sac as related to growth. This is 

 attested by the remainder of the same sen- 

 tence : "indeed, the embryo in question is 

 slightly larger than its twin . . . ," and again 

 further on : "In the earliest human embryos 

 known, when it might be of real use, it (the 

 yolk sac) is a simple entodermal sac containing 

 masses of coagulum; growth to a conspicuous 

 size is attained relatively late, long after ade- 

 quate nutritional relations with the mother 

 have been thoroughly established." Little did 

 I suspect that any one would infer an intended 

 reference to initial morphological development 

 in its strict sense. Of course, the gut and 

 allantois had to form from entoderm some- 

 where, and the yolk sac, broadly speaking, is 

 the imdoubted source, yet it is entirely con- 

 ceivable that essentially all the yolk sac, as a 

 significant sac, might be dispensed with and 

 still the gut would arise from entodenn which 

 for the most part normally forms its roof. 

 With this in mind I wrote that the fission "was 

 presumably such that one received all, or 

 essentially all, the cells destined to form a yolk 

 sac," etc. Again, that I recognized the possi- 

 bility of a rudimentary or abortive sac is seen 

 in a later sentence : "That tiny vascular 

 anlages of yolk-sac ancestry actually existed 

 ... is of coui-se conceivable." 

 In short, my aim was remote from the heresy 

 of denying the gut an entodermal, yolk-sac 

 origin; on the contrary it was to re-emphasize 

 from the functional side precisely what Pro- 

 fessor Lewis has designated as a platitude: 

 "But it is universally recognized that the yolk 

 sac does its work in early stages, and . . . usu- 

 ally persists as a functionless rudiment until 



