Talk:Paper - On the presence of a series of ectodermal placodes in the head region of a sparrow embryo (1928)

bON THE PRESENCE OF A SERIES OF ECTODERMAL PLACODES IN THE HEAD REGION OF A SPARROW EMBRYO

By FRANK GOLDBY, B.A. (CAnras.) Formerly “Frank Smart” Student of Gonville and Caius College

Tue following observations were made while working in the Embryological Laboratory of the Anatomy School in Cambridge under the direction of Prof. J. T. Wilson in 1924, during the tenure of a Studentship in Gonville and Caius College.

The specimens were shown at the May meeting of the Anatomical Society of Great Britain and Ireland in Sheffield in that year, and a brief description appeared in the Proceedings of the Society for May 1924, p. 104. The main object of this paper is to furnish illustrations of the structures in question.

The observations were made upon a series of transverse sections of a spar- row embryo of a stage corresponding roughly to that of a chick embryo of about 40 hours. The series was incomplete at the caudal end of the embryo and unfortunately the histological condition of the specimen was not entirely satis- factory, so that it was impossible to ascertain with precision the possible under- lying neural relationships of the placodes. Nevertheless, the actual structural features presented by the members of this extraordinary series of epidermal placodes are so clearly and definitely marked, and their occurrence in an amniote represents such a striking and unprecedented phenomenon, that it seems worthy of special attention and record.

The placodes in question are small areas of more or less columnar ecto- dermal cells situated in a dorso-lateral position along the head. They appear definitely to lie in series with the auditory placode, two of them being caudal to it and the others more cranially (rostrally) placed. Not the least extra- ordinary fact concerning them is that they are present only on one side of the head. Their relative positions—and also their relative sizes—are roughly indicated in the diagram in fig. 1, founded upon Lillie’s diagram of the head region in a 40-hour chick embryo.

The placodes may be described individually as follows:

Placode I (fig. 2). This consists of a well-defined and sharply cupped placode consisting of cuboidal cells which are distinctly larger than the cells of the neighbouring epidermis. It is situated alongside the mid-brain region about its middle. Underneath it lie a number of cells which can be traced along the side of the mid-brain towards its rostral limit. These cells may possibly 136 Frank Goldby

V Ill I

V IT VI ap -

NC FG MB FB Fig. 1. Median sagittal section of a 40-hour chick embryo (slightly modified from Lillie). AP, auditory placode; F B, fore-brain; F G, fore-gut; N C, notochord.

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Figs. 2-5. Transverse sections across head of sparrow embryo of a stage corresponding to fig. 1. C, constriction between fore-brain and mid-brain; D A, dorsal aorta; FG, fore-gut; H F A, head fold of amnion; PI-VI, placodes I-VI; SI, cranial end of first somite; VC L, vena

capitis lateralis; V 2-3, trigeminal neural crest tissue. Ectodermal Placodes in Head Region of Sparrow Embryo 137

represent the ophthalmic division of the trigeminus. There is no evidence of any proliferation of cells from the placode joining this group of cells.

Placode II. This lies close to the caudal boundary of the mid-brain. It is much smaller than placode I, but otherwise it is very little different. No special cell-group underlies it.

Fig. 4. Fig. 5.

Placode III (fig. 3). This placode lies cranially to the first visceral pouch. It is the largest of the series and, like the first, it is sharply cupped. The concave placode consists of well-marked columnar cells sharply defined basally. Be- 138 Frank Goldby

neath it lies a cell-proliferation which may represent a portion of the trigeminal neural crest. Yet here again there does not appear to be any morphological relation, other than contiguity, between the placode and the presumable crest- cells.

Placode IV (fig. 4). This small but quite distinct placode also lies just anterior to the first visceral pouch. There is no evidence of any ganglionic proliferation in relation with it.

Placodes V and VI. These, although in series with the others, lie caudal to the auditory placode. They are smaller and less well-formed than the anterior members of the series. Placode VI is fairly definite (fig. 5). It lies opposite the cranial end of the first somite. (In the earlier note in Proc. Anat. Soc., 1924, p. 104, this placode was erroneously described as lying lateral to the gang- lion of the [Xth nerve. In that note also a different mode of designation was employed for the placodes.) Placode V lies above the second visceral pouch. It is the only one of the series about which there can be any real doubt. It is both small and indefinite and might possibly be only an artificial indentation, though it is believed to be a rather imperfectly formed member of the series.

The sectional series further affords some indication that placodes V and VI form portions of a ridge of somewhat thickened ectoderm extending from placode V caudally through the VIth and becoming lost a few sections further on. It is however not very well marked, nor quite continuous, and its existence would require confirmation in better material.

Except in its much larger size the auditory placode does not differ markedly from the placodes above described. Its constituent cells, however, cannot be distinguished so clearly, though this is probably only due to more active pro- liferation.

According to their position the placodes might be classified, as in the former note, as follows:

Premandibular Placodes I and II.

Mandibular Placodes III and IV.

Hyoidean The auditory placode and possibly placode V. Branchial Placode VI.

But any real morphological relation to the arches of the head or even to the ganglia is very questionable. The facts that (a) the placodes are unilateral in this, the single observed case, that (b) they exhibit such irregularity in size among themselves, and (c) that so far as can be seen they correspond to no adult structure, all suggest that they are vestigial in character. Their general position, and their arrangement in obvious series with the auditory placode, give colour to the further suggestion that they represent traces of the acoustico- lateral system of anamniota.

In conclusion I wish to express my gratitude to Prof. Wilson for the help and advice which he has given me. My thanks are due also to Mr Walter Cal- eott, from whose photomicrographs the illustrative figures were traced.