428 A Three Weeks' Human Embryo 



pound eyes is really cephalad of that connected with the antenna, now 

 proved to function as organs of smell. This fact seems to fit into the 

 finding given above on the serial order of parts. 



Total folds. — In Figs. 3, 4, it is seen that the neural tube is divided 

 more or less clearly into lobules and these again into folds. Those now 

 under consideration are not the total folds of the cerebrum considered by 

 some authors as transitory fissures and by others as artifacts. Indeed, 

 some of them antedate the distinct formation of a cerebrvim, some being 

 formed in early human specimens with open neural plates (No. 12 of 

 Johns Hopkins University Collection, and No. 714 of the Harvard 

 University Embryological Collection). In the present study, the cere- 

 brum itself is the name applied to one of these total folds. 



Nor are they newly recognized structures. Bischoff ^^ in 1845 published 

 a minute figure of the brain of an embryo dog showing such folds in the 

 oblongata. 



Total Folds in the Immammalia. — Orr," in 1887, found in the lizard 

 a series of such total folds. McClure '" followed with similar results. 

 Locy's ^° remarkable dissections of shark and also of chick and of Ambly- 

 stoma show the beginning of these folds as marginal structures before the 

 mesoderm had reached the parts, and therefore indicating the segmenta- 

 tion of the epidermis antecedent to that of the mesoderm. Locy's results 

 have been questioned by Neal,*" but in going over some of the same ground 

 it seems to me that Locy's observations are well founded. The careful 

 confirmatory work done in Locy's laboratory by Hill " seem to put the 

 essential points beyond controversy. He shows 3 neuromeres in the fore- 

 brain, 2 in the mid-brain, and G in the hind-brain. 



Orr, McClure, and Locy call the folds, neuromeres. This term is here 

 avoided because it leads too far afield into a consideration of related 

 questions of theory and fact concerning nerve distribution and meso- 

 dermic segments on which the literature is extensive and well-known. 



Total Folds in Mammals. — In mammals, since the time of Bischoff,^* 

 figures of such folds appear occasionally in literature. Mihalkovics " 

 shows folds in the rabbit's oblongata; Prenant" in that of the pig. 



^"von Bischoff, T. L. W., Entwicklungsgeschichte des Hunde-eies. Braun- 

 schweig, 1845. 

 ^■Orr, Henry, Jour. Morph., I, 1887. 

 "" McClure, Chas. F., Jour. Morph., IV, 1890. 

 =°Locy, W. A., Jour. Morph., XI, 1895. 



" Neal, H. V., Bull. Mus. Comp. Zool., Harvard Coll., XXXI, 1898. 

 *^Hill, C, Zool. Jahrb., Abth. f. Anat. u. Ontog. d. Thiere, XIII, 1900. 

 ^Mihalkovics, V. von, Entwicklungsgeschichte des Gehirns. Leipzig, 1877. 

 *' Prenant, A., Soc. de la science de Nancy, Bull., Ser. 2, IX, 1889. 



