CHRONOLOGY OF HUMAN DEVELOPMENT. 877 



pachydermata, cetacea, solidungula, camelidae). In the ruminants, the villi are arranged in 

 groups or cotyledons, which grow into the uterine mucous membrane, from which they are 

 pulled out at birth, (b) In the deciduata, there is such a firm union between the chorionic 

 villi with the uterine mucous membrane, that the uterine part of the placenta comes away 

 with the fcetal part at birth. In this case the placenta is either zonary (carnivora, pinnipedia, 

 elephant) or discoid (apes, insectivora, edentata, rodentia). 



446. CHRONOLOGY OF HUMAN DEVELOPMENT. Development during the 1st Month. 

 At the 12-13th day the ovum is saccular (5 "5 mm. and 3 mm. in diameter) ; there is simply 

 the blastodermic vesicle, with the blastoderm at one part, consisting of two layers ; the zona 

 pellucida beset with small villi (Reichert). At the 15th-16th day the ovum (5-6 mm.) is 

 covered with simple cylindrical villi. The zona pellucida consists of embryonic connective- 

 tissue covered with a layer of flattened epithelium. The primitive groove and the lamina? 

 dorsales appear. Then follows the stage when the allantois is first formed. At the 15th-18th 

 day Coste investigated an ovum. It was 13*2 mm. long, with small branched villi ; the 

 embryo itself was 2*2 mm. long, of a curved form, and with a moderately enlarged cephalic end. 

 The amnion, umbilical vesicle with a wide vitelline duct,, and the allantois were developed, the 

 last already united to the false amnion. The S-shaped heart lies in the cardiac cavity, shows a 

 cavity and a bulbus aortas, but neither auricles nor ventricles. The visceral arches and clefts 

 are indicated, but they are not perforated. The omphalo-mesenteric vessels forming the first 

 circulation on the umbilical vesicle are developed, the duct (vitelline) is still quite open, and 

 two primitive aortas run in front of the protovertebrse. The allantois attached to the fcetal 

 membranes is provided with blood-vessels. The two omphalo-mesenteric veins unite with the 

 two umbilical veins, and pass to the venous end of the heart. The mouth is in process of 

 formation. The limbs and sense-organs absent : the Wolffian bodies probably present. 



At the 20th day all the visceral arches are formed, and the clefts are perforated. The mid- 

 brain forms the highest part of the brain, while the two auricles appear in the heart. The con- 

 nection with the umbilical vesicle is still moderately wide. The embryo is 2*6-3 '3-4 mm. 

 long, while the head is turned to one side (His). At a slightly later period the temporal and 

 cervical flexures take place, and the hemispheres appear more prominently ; the vitelline duct 

 is narrowed, the position of the liver is indicated, while the limbs are still absent (His). 



At the 21st day the ovum is 13 mm. long and the embryo 4-4*5 mm.; the umbilical vesicle 

 2*2 mm., and the intestine almost closed. Three branchial clefts, Wolffian bodies laid down, 

 and the first appearance of the limbs, three cerebral vesicles, auditory capsules present (E. 

 Wagner). Coste also observed, in addition, the nasal pits, eye, the opening for the mouth, 

 with the frontal and superior maxillary processes, the heart with two ventricles and two 

 auricles. 



End of the 1st Month. The embryos of 25-28 days are characterised by the distinctly 

 stalked condition of the umbilical vesicle and the distinct presence of limbs. Size of the ovum, 

 17 '6 mm.; embryo, 13 mm.; umbilical vesicle, 5*5 mm., with blood-vessels. 



2nd Month. The embryos of 28-35 days are more elongated, and all the branchial clefts are 

 closed except the first. The allantois has now only three vessels, as the right umbilical vein 

 is obliterated. At the 5th week the nasal pits are united with the angle of the mouth by 

 furrows, which close to form canals at the 6th week ( Toldt). At 35-42 days the nasal and oral 

 orifices are separated, the face is flat, the limbs show three divisions, the toes are not so sharply 

 defined as the fingers. The outer ear appears as a low projection at the 7th week. The 

 Wolffian bodies are much reduced in size. Length of body at 7th to 8th week, 1 '6-4-1 cm. 



End of the 2nd Month. Ovum, 6| cm.; villi, 1*3 mm. long; the circulation on the 

 umbilical vesicle has disappeared ; embryo, 26 mm. long, and weighs 4 grammes. Eyelids and 

 nose present, umbilical cord 8 mm. long, abdominal cavity closed, ossification beginning in the 

 lower jaw, clavicle, ribs, bodies of the vertebrae ; sex indistinct, kidneys laid down. 



3rd Month. Ovum as large as a goose's egg, beginning of the placenta, embryo 7-9 cm., 

 weighing 20 grammes, and is now spoken of as a foetus. External ear well formed, umbilical 

 cord 7 cm. long. Beginning of the difference between the sexes in the external genitals, 

 umbilicus in the lower fourth of the linea alba. 



4th Month. Foetus, 17 cm. long, weighing 120 grammes, sex distinct, hair and nails begin- 

 ning to be formed, placenta weighs 80 grammes, umbilical cord 19 cm. long, umbilicus above 

 the lowest fourth of the linea alba, contractions or movements of the limbs, meconium in the 

 intestine, skin with blood-vessels shining through it, eyelids closed. 



5th Month. Foetus, length of body, 9 7-14*7 cm., "total length 18 to 28 cm., weighing 284 

 grammes ; hair on the head and lanugo distinct ; skin still somewhat red and thin, and covered 

 with vernix caseosa ( 287, 2), is less transparent ; weight of placenta, 178 grammes ; umbilical 

 cord, 31 cm. long. 



6th Month. Foetus, length of body, 15-18*7, total length, 29-37 cm., weighing 634 

 grammes ; lanugo more abundant ; vernix more abundant ; testicles in the abdomen ; pupillary 

 membrane and eyelashes present ; meconium in the large intestine. 



7th Month. Foetus, length of body, 18-22*8, total length, 35-38 cm., weighing 1218 



