Paper - A Human Embryo Twenty-Six Days Old
Introduction
SEVERAL years ago Dr. C. 0. Miller of the Johns Hopkins University gave me a very young human embryo which was so well preserved and so perfect in all respects that it justified a very careful study. He very kindly has procured for me the following history.
- ‘‘The woman, twenty-nine years old, had been married nine years, and had always menstruated regularly every twenty-eight days, the period each time lasting three days. She had given birth to four healthy children, the last having been born January I, 1888. Her last menstrual period began on October 6, 1888, and ended on the 9th. Her next menstrual period should have begun on November 3, but on account of its falling out, she concluded that she was pregnant, and, on November 20,began taking large doses of ergot, which she had repeatedly taken to produce abortion in earlier pregnancies, but with no result, Several days later she applied to a professional abortionist who used instruments, after which she had a continuous metrorrhagia, and called for me to attend her. On November 27, just fifty-two days after the beginning of the last menstrual period, the unbroken ovum came away. It was kept in a cool place for three hours, and then without opening placed in eighty per cent alcohol.”
When the specimen came into my hands it was found covered with villi two or three millimetres in length, without which it measured 22 mm. in diameter. Upon opening it I found that the embryo had been hardened without any irregular shrinkage. A year later it was shown by staining a portion of the membranes that the cells were preserved excellently; and the embryo was then stained with alum carmine, imbedded in paraffin, and cut into sections at right angles to the branchial arches 15 microns thick.
Age
The nape-breech length measured 7 mm. and the vertex- breech 6 mm. The study of mammalian embryos, as well as a series of human embryos, tells us that this embryo cannot be over a month old. From the results of post-mortem examinations of women shortly before the' beginning of menstruation, Bischoff, Williams, Dalton, Leopold, and others place ovulation two or three days before the beginning of menstruation.' Espe- cially on account of the study of several cases in which the earliest possible cohabitation took place a week or two after the last menstrual period, embryologists and gynecologists reckon the duration of pregnancy from the beginning of the first period which has fallen out.
So in order to estimate more accurately the age of this em- bryo, we must subtract twenty-eight from the time which has elapsed since the beginning of the last period (fifty-two days), and add two for the time between ovulation and menstruation. The shape and size of this embryo correspond with that described by others as the fourth week, and twenty-six days is in all proba- bility its age.
External Form
The embryo is flexed upon itself, forming almost a circle (Pl. XXIX., Fig. I). The head shows the outline of the brain within, and also a marked elevation over the region of the Gasserian ganglion. The nasal pit is a large shallow depression, being well exposed on both sides. T h e lense is small, and is surrounded by a groove which is continued between the superior maxillary process and nasal pit.
Three branchial arches are visible on the right side, and four on the left. The ventral end of the first is bulbous, while from its dorsal end the superior maxillary process arises. The second is also bulbous on its ventral end, the major portion of the trunk hanging over the third arch. This is the embryonic operculum which will finally close the sinus praecervicalis. The third arch lies more towards the median line, that is, it is within the sinus praecervicalis. The fourth arch is visible only on the left side; it lies deep in the sinus praecervicalis, and is almost covered by the third arch.
The clefts are irregular in shape, as shown in the figure; and the first, second, and third show marked depressions at their dorsal ends, which indicate the blending of the ectoderm with the seventh, ninth, and tenth nerves.
On the dorsal side of the branchial region, alike on both sides of the head, there is a marked depression which lies immediately over the otic vesicle. The vesicle, however, is fully separated from the ectoderm.
The protovertebrae are more marked on the right than the left side ; twenty-seven on the right, and twenty-four on the left. On the right only the last seven cervical, all the dorsal and lumbar, and five sacral are visible; while on the left two occipital, all the cervical and dorsal, and but two lumbar are seen. The sections, however, show that the muscle plates are the same on both sides.
The extremities are well marked, the anterior being some- what larger than the posterior. The anterior on the right side is flat, and bent directly towards the median line ; while on the left it hangs away from the mouth. The posterior on the right is bent towards the head, and on the left side it is simply an oval mound.
Upon the body proper there are three marked elevations: two for the heart, and one for the liver. In general, these elevations are the same on both sides.
The umbilical cord is large, and lies on the left side of the body, as described by Waldeyerl and by Janoiik.2 In most embryos described it lies on the right side. The cord is short, and midway between the embryo and its attachment to the chorion it shows a decided enlargement. The umbilical vesicle is large, measuring 7 mm. in length and 5 in diameter.
Method of Study
The embryo was stained with alum carmine, and cut into serial sections I 5 microns thick. Although thinner sections undoubtedly would have been better to study the histology, for my purpose I desired sections in no way distorted. On account of greater ease in studying the sections, I cut the embryo at right angles to the nape-breech length, at the same time striking the branchial arches nearly at right angles. In all there were 351 perfect sections, none lost, and only two or three slightly distorted. The sections were enlarged 662 diameters, which at the same time increased their thickness to I mm. Every second section was now drawn on wax plates 2 mm. thick, and the external outline of the section cut out. Before working the interior of the enlarged sections, they were carefully piled, in order to obtain the form of the exterior of the embryo. Next, I made a plaster mould of the pile of plates, in order to keep them in position. The plates were constantly kept within the half-mould, in order to keep the model from becoming distorted. This latter procedure proved of great value, as the plates were cut into pieces in order to model the various organs. All the wax representing the tissue between the organs and the exterior of the body on the left side was removed, exposing the organs as shown in the figure on Plate XXX. Then the organs were freed from the opposite side, and the pieces of wax blended so as to isolate each organ by itself.
The body cavity was modelled as a corrosion preparation, by drawing its outline on a second set of plates, and removing all the wax representing the body cavity, then piling the plates again, and finally casting the whole with Wood's metal. The metal was next smoothed and imbedded in plaster of paris, from which it was removed by boiling. The plaster mould was now cast with solder, and the wax broken off. By this method a metal cast of the whole ccelom was obtained.
The shrinkage while imbedding in paraffin was slightly over ten per cent, so our model, which represents the section of the embryo enlarged 662/8 times, is but sixty times larger than the specimen while in alcohol. All the measurements I give have been reduced to correspond with those of the alcoholic specimen.
Central Nervous System
When the neural tube is straightened it measures from end to end 17mm. with a diameter of 1.5 mm. for the fore-brain, and 0.5 mm. at the point between the posterior extremities. Its general shape is shown, Plate XXX., which corresponds quite well with His’s BY.,.^ On the exterior it is plainly shown that the cerebrum and optic vesicle are attached to the fore-brain. The cerebral hemispheres are represented on both sides by oval projections from the fore-brain, extending somewhat over the surrounding tissue on the mid-brain side, and under the eye. Viewed from the median side the pit represents a stage midway between His’sBY.andK.0.2On the median line between the two hemispheres there is a fold of medullary tube-wall which extends more than a third across the cerebrum, and has a tendency to cut it in half. It measures 0.4 mm. in perpendicular, and projects 0.16mm. into the fore-brain vesicle. Compared with His’s figures, this is undoubtedly the epithelial covering of the choroid plexus.
Towards the mouth from the cerebrum there is the opening into the optic vesicle. It is triangular in shape, with the base on the oral side, and the apex pointing towards the inter-brain. The opening within the stem of the optic vesicle is round, and ends as a circle about the secondary ocular vesicle. Viewed from the outside, the secondary ocular vesicle is a round pit, in which lies the lense. There is no slit on the oral side of the stem (Plate XXX., Fig. I).
The inter-brain shows a marked constriction in its middle, both on‘ the outside and also within its lumen. This undoubt- edly was caused by a shrinkage when the specimen was hardened.
As the mid-brain and hind-brain are approached, the walls gradually become thicker and thicker, until the after-brain is reached, when the ventral side alone increases, while the dorsal walls become very thin. From the origin of the facial nerve to the origin of the pneumogastric, the walls of the dorsal half of the neural tube are very thin. From now on, the thickness of the walls of the tube, with the exception of the extreme dorsal and ventral sides,l is much the same in transverse section, the thickness gradually diminishing as the tail is approached.
Cranial Nerves
The olfactory pit is sharply defined and is composed of five or six layers of cells. Throughout its extent cell divisions are present, most numerous, however, at its concave oral side. In this region there are marked pyramidal cells with their base towards the outside of the body and their apices pointing towards the brain. These cells undoubtedly mark the beginning of the olfactory nerve as pointed out by His. In Amblystoma and Necturus these cells are much more pronounced and can be traced in the various stages from the olfactory pit to the brain.
There is as yet no indication of a permanent optic nerve. The primary optic vesicle still communicates with the fore-brain while the distal or rod-and-cone layer of the secondary vesicle shows peculiar changes. The layer is about twice as thick as the proximal or pigment layer, and in both these are nuclear figures. The location of these cells, which are dividing, is on the margin which corresponds with the layer about the central canal in the spinal cord. The pigment layer is about five cells thick, while the rod-and-cone layer is about eight.
The rod-and-cone layer is composed of two distinct zones, - a distal or hyaline and a proximal or cellular. The hyaline zone lies next to the lense and seems to be composed of cilia, all being directed towards a centre lying within the lense. The granular zone is composed in great part of round cells between which are many bipolar and unipolar cells. The unipolar cells are more numerous than the bipolar and project, with their pole, towards the position which is later to be rods and cones. The bipolar send one pole in the same direction and the other into the hyaline layer.
At a point in the stem of the optic vesicle nearest the mouth there enters a vessel which is undoubtedly the arteria centralis retinz. As this perforates the hyaline layer, the “cilia” become shorter, but are in no way directed towards this artery, ie.thefuturedirectionoftheopticnerve. Nodoubtthislayer is identical with the peripheral veil as described by His.1
It is extremely difficult to locate the origin of the third nerve. At the floor of the mid-brain there is a suspicious spot composed of several dozen cells which are somewhat separated from the remaining cells and lie partly within the terminal veil (Pl. XXX., Fig. 1, 111.). No nerve fibres extend from the brain into the surrounding tissue.
The trochlear nerve is well marked as a small group of cells in the ventral wall of the isthmus between the mid-brain and hind-brain. The cells lie just under the terminal veil, and each sends a single short pole towards the dorsal part of the brain. They extend but half-way around the tube.
The Gasserian ganglion with its three branches marks the trifacial. Upon its ophthalmic branch a small group of cells indicates the ciliary ganglion. From the Gasserian ganglion numerous fibres enter the hind-brain as its sensory root. The motor root arises more ventral from a large group of cells and passes as a large bundle of fibres into the inferior maxillary branch of the nerve.
The sixth nerve is representedasa small group of cells, dorsal, but somewhat aboral from the first branchial cleft. None of the cells send prolongations from the brain to form a distinct nerve, but all of the unipolar motor cells are pointed in one direction.
It is extremely difficult to isolate the facial nerve from the auditory nerve ganglion. Following it from t h e second branchial arch it passes through the heart of the acoustico-facial ganglioh, and after entering the neural tube passes towards the ventral side of the same, making an arch around the ganglion of the sixth to take its origin near the median line.
The acoustic ganglion extends from the facial to the auditory vesicle, to which it is adherent, and then with the facial nerve sends twigs into the after-brain. The auditory vesicle is olive- shaped, is placed at right angles to the after-brain, and from its dorsal end there is a marked prolongation, the beginning of the aquaeductus vestibuli. The walls are of quite even thickness throughout, and the lumen is of the same general shape as the vesicle.
Between the auditory vesicle and the after-brain lies the upper ganglion of the glosso-pharyngeal which receives fibres from a group of cells lying in the floor of the after-brain, and sends a bundle of fibres more dorsally into the same. On the ventral side there is a bundle of nerves which communicates with a second ganglion, the ganglion petrosum. The ganglion petrosum is in direct continuity with a slight invagination of ectoderm at the dorsal part of the second branchial groove, sends a branch into the third arch, and communicates with the ganglion jugulare of the vagus.
The vagus is composed of two enormous ganglia, as shown in P1.XXX. The two ganglia are united by a band of cells, and from the ganglion nodosum a branch passes to the aboral and lateral side of the fourth branchial arch. At the central end of the nerve numerous branches pass into the after-brain. The ganglion jugulare receives at once twigs from the accessory nerve, which soon arrange themselves into a bundle to become fully separated before the ganglion nodosum is reached.
The accessory nerve arises as a row of bundles between the vagus and first cervical, and emerges from the after-brain midway between its dorsal and ventral walls. As the twigs ap- proach the first cervical nerve, the origin becomes more ventral, and are continuous with the ventral root of this nerve. Although the rudimentary ganglion, first described by Froriep as the ganglion of the accessory, has been verified by His for the human embryo, I cannot find any trace of it in this specimen. I have had no difficultyin finding it in dog and cat embryos, and there- foremust conclude that it is wanting in this embryo. This is what we should expect to find from time to time, especially in an organ which is in process of degeneration.
The hypoglossal nerve arises as a group of fibres parallel to but more ventral than the accessory. The bundles are arranged to correspond with the myotomes of the head, and on the aboral side arise in common with the accessory and the first cervical nerve.
The Spinal Nerves
The spinal nerves are all distinctly marked by a large dorsal ganglion which sends small bundles of fibres into the cord, and a ventral root which arises from the motor cells in the anterior horn of the same. The ganglia are largest in the cervical region, and gradually diminish in size as the tail is approached. The eight cervical nerves are united by anastomoses, which are destined to form the cervical and brachial plexuses respectively. The distal ends of the upper eight dorsal nerves are divided into two branches. Beyond these the remaining dorsal, lumbar, and sacral nerves end as a single branch. In all, twenty-nine spinal nerves can be identified ; i.e. eight cervical, twelve dorsal, five lumbar, and four sacral. Beyond this there is a group of myotomes, which towards the tip of the tail run into one another. In this region the dorsal ganglia are-not fully separated from the spinal cord ; in fact, the whole seems yet to be blended with the ectoderm.
